Defeated Republican presidential candidate Mitt Romney has urged his supporters to avoid "partisan bickering and political posturing".

In his concession speech given to a subdued ball room in Boston, Massachusetts, Romney said: "Thank you my friends, thank you so very much. I have just called President Obama to congratulate him on his victory. His supporters and his campaign also deserve congratulations.

"I wish all of them well, but particularly the president, the first lady and their daughters.

"This is a time of great challenges for America, and I pray that the president will be successful in guiding our nation."

Ryan also thanked his vice presidential running mate, congressman Paul Ryan, saying he hoped he would continue to use "his intellect and his hard work and his commitment to principle" for the good of the nation.

"The nation, as you know, is at a critical point. At a time like this we can't risk partisan bickering and political posturing," he said.

"Our leaders have to reach across the aisle to do the people's work and we citizens also have to rise to the occasion."

Governor Romney said: "I believe in America. I believe in the people of America, and I ran for office because I am concerned about America.

"This election is over, but our principles endure. I believe that the principles upon which this nation are the only sure guide to a resurgent economy and to a new greatness."

Romney added: "Thank you and God bless America. You guys are the best, thank you so much. Thanks guys."

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  • Republican Presidential Candidate Mitt Romney Holds Election Night Gathering In Boston

    BOSTON, MA - NOVEMBER 07: Republican presidential candidate, Mitt Romney, speaks at the podium as he concedes the presidency during Mitt Romney's campaign election night event at the Boston Convention & Exhibition Center on November 7, 2012 in Boston, Massachusetts. After voters went to the polls in the heavily contested presidential race, networks projected incumbent U.S. President Barack Obama has won re-election against Republican candidate, former Massachusetts Gov. Mitt Romney. (Photo by Matthew Cavanaugh/Getty Images)

  • Republican Presidential Candidate Mitt Romney Holds Election Night Gathering In Boston

    BOSTON, MA - NOVEMBER 07: Republican presidential candidate, Mitt Romney, speaks at the podium as he concedes the presidency during Mitt Romney's campaign election night event at the Boston Convention & Exhibition Center on November 7, 2012 in Boston, Massachusetts. After voters went to the polls in the heavily contested presidential race, networks projected incumbent U.S. President Barack Obama has won re-election against Republican candidate, former Massachusetts Gov. Mitt Romney. (Photo by Matthew Cavanaugh/Getty Images)

  • Republican Presidential Candidate Mitt Romney Holds Election Night Gathering In Boston

    BOSTON, MA - NOVEMBER 07: Republican presidential candidate, Mitt Romney greets supporters as he concedes the presidency during his campaign election night event at the Boston Convention & Exhibition Center on November 7, 2012 in Boston, Massachusetts. After voters went to the polls in the heavily contested presidential race, networks projected incumbent U.S. President Barack Obama has won re-election against Republican candidate, former Massachusetts Gov. Mitt Romney. (Photo by Don Emmert-Pool/Getty Images)

  • Republican Presidential Candidate Mitt Romney Holds Election Night Gathering In Boston

    BOSTON, MA - NOVEMBER 07: Republican presidential candidate, Mitt Romney, speaks at the podium as he concedes the presidency during Mitt Romney's campaign election night event at the Boston Convention & Exhibition Center on November 7, 2012 in Boston, Massachusetts. After voters went to the polls in the heavily contested presidential race, networks projected incumbent U.S. President Barack Obama has won re-election against Republican candidate, former Massachusetts Gov. Mitt Romney. (Photo by Matthew Cavanaugh/Getty Images)

  • Republican Presidential Candidate Mitt Romney Holds Election Night Gathering In Boston

    BOSTON, MA - NOVEMBER 07: Republican presidential candidate, Mitt Romney, speaks at the podium as he concedes the presidency during Mitt Romney's campaign election night event at the Boston Convention & Exhibition Center on November 7, 2012 in Boston, Massachusetts. After voters went to the polls in the heavily contested presidential race, networks projected incumbent U.S. President Barack Obama has won re-election against Republican candidate, former Massachusetts Gov. Mitt Romney. (Photo by Matthew Cavanaugh/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Ann Romney (2nd-R) stands on stage with her family after husband, Republican presidential candidate Mitt Romney, conceded defeat to US President Barack Obama November 7, 2012 in Boston, Massachusetts. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Republican presidential candidate Mitt Romney arrives on stage on election night November 7, 2012 in Boston, Massachusetts, moments before conceding defeat to US President Barack Obama in the 2012 US presidential election. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    US Republican presidential candidate Mitt Romney (L), his wife Ann (2nd L), running mate Paul Ryan (2nd R) and his wife Janna appear on stage after Romney conceded defeat to President Barack Obama on election night on November 7, 2012 in Boston. Obama swept to re-election, forging history again by transcending a slow economic recovery and the high unemployment which haunted his first term to beat Republican Mitt Romney. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    US Republican presidential candidate Mitt Romney and his wife Ann wave after Romney conceded defeat to President Barack Obama on November 7, 2012 in Boston. Obama swept to re-election, forging history again by transcending a slow economic recovery and the high unemployment which haunted his first term to beat Republican Mitt Romney. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    US Republican presidential candidate Mitt Romney and his wife Ann wave after Romney conceded defeat to President Barack Obama on November 7, 2012 in Boston. Obama swept to re-election, forging history again by transcending a slow economic recovery and the high unemployment which haunted his first term to beat Republican Mitt Romney. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Republican presidential candidate Mitt Romney arrives on stage on election night November 7, 2012 in Boston, Massachusetts, moments before conceding defeat to US President Barack Obama in the 2012 US presidential election. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Republican presidential candidate Mitt Romney arrives on stage on election night November 7, 2012 in Boston, Massachusetts, moments before conceding defeat to US President Barack Obama in the 2012 US presidential election. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Republican presidential candidate Mitt Romney arrives on stage on election night November 7, 2012 in Boston, Massachusetts, moments before conceding defeat to US President Barack Obama in the 2012 US presidential election. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • US-VOTE-2012-ELECTION-ROMNEY

    Republican presidential candidate Mitt Romney arrives on stage on election night November 7, 2012 in Boston, Massachusetts, moments before conceding defeat to US President Barack Obama in the 2012 US presidential election. AFP PHOTO/EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to his election night rally, Wednesday, Nov. 7, 2012, in Boston. President Obama defeated Republican challenger former Massachusetts Gov. Mitt Romney. (AP Photo/David Goldman)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to his election night rally, Wednesday, Nov. 7, 2012, in Boston. President Obama defeated Republican challenger former Massachusetts Gov. Mitt Romney. (AP Photo/David Goldman)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to give his concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to address supporters during his election night rally, Wednesday, Nov. 7, 2012, in Boston. President Obama defeated Republican challenger former Massachusetts Gov. Mitt Romney. (AP Photo/Elise Amendola)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney waves to supporters before conceding at his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney waves to supporters during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney takes the podium to give his concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to his election night rally, Wednesday, Nov. 7, 2012, in Boston. President Obama defeated Republican challenger former Massachusetts Gov. Mitt Romney. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney, Paul Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney, left, wife Ann Romney, center, and Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., stand on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia)

  • Mitt Romney, Ann Romney,

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney, right kisses his wife Ann Romney during Romney's election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney waves to supporters during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Mary Altaffer)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney gives a thumbs up as he arrives to give his concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney gestures to supporters during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney,

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann walk off the stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney, embraces with his wife Ann Romney, left, as Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., and his wife Janna walk onto the stage at left, during Romney's election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann, left, and Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., and his wife Janna, right, are joined by their families on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • Mitt Romney, Ann Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann, center, are joined by their sons and their wives on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney embraces Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., as Ann Romney and Janna Ryan embrace after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Mary Altaffer)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney embraces Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., as Ann Romney and Janna Ryan embrace after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston.(AP Photo/David Goldman)

  • Mitt Romney

    Republican presidential candidate former Massachusetts Gov. Mitt Romney arrives to give his concession speech at his campaign election night event at the Boston Convention & Exhibition Center Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Rick Wilking, Pool)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney embraces Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., as Ann Romney and Janna Ryan embrace after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Mary Altaffer)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann, left, and Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., and his wife Janna, right, are joined by their families on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann stand on the stage with Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., and his wife Janna after Mitt Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann are joined by their son Craig Romney, right, and other family members on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • Mitt Romney

    Republican presidential candidate former Massachusetts Gov. Mitt Romney leaves the podium giving his concession speech at his campaign election night event at the Boston Convention & Exhibition Center Wednesday, Nov. 7, 2012, in Boston. His wife Ann waits for him in red and running mate Rep. Paul Ryan, R-Wis., and his wife Janna wait at lower right. (AP Photo/Rick Wilking, Pool)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Standing with their families, Republican presidential candidate and former Massachusetts Gov. Mitt Romney and wife Ann Romney, left, stand with Republican vice presidential candidate Paul Ryan, second right, and his wife Janna, third right, as they wave to supporters after Romney gave his concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney, Ann Romney, Paul Ryan,

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney walks behind his wife Ann to meet Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., on stage after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia)

  • Mitt Romney, Ann Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann Romney wave to supporters after Romney conceded the race at his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney waves to supporters during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Elise Amendola)

  • As Mitt Romney delivers his concession speech, people cheer during a Democratic election night watch party Tuesday, Nov. 6, 2012, in Salt Lake City. (AP Photo/Jim Urquhart)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney and his wife Ann Romney, left, and Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., and his wife Janna, right, wave to supporters after Romney conceded at his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia)

  • Mitt Romney, Ann Romney, Craig Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney embraces with his son Craig Romney as Ann Romney looks on after Romney conceded the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/David Goldman)

  • Mitt Romney, Ann Romney, Paul Ryan, Janna Ryan

    Standing with their families, Republican presidential candidate and former Massachusetts Gov. Mitt Romney stands with his wife Ann, and Republican vice presidential candidate Rep. Paul Ryan, R-Wis., second right, stands with his wife Janna after Romney's concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney walks away from the podium after conceding the race during his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia, Pool)

  • Mitt Romney, Ann Romney

    Republican presidential candidate and former Massachusetts Gov. Mitt Romney walks off stage with his wife Ann Romney after he arrives gave his concession speech at his election night rally in Boston, Wednesday, Nov. 7, 2012. (AP Photo/Charles Dharapak)

  • Mitt Romney

    Republican presidential candidate former Massachusetts Gov. Mitt Romney and his wife Ann and members of his family wave after his concession speech at his campaign election night event at the Boston Convention & Exhibition Center Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Rick Wilking, Pool)

  • Paul Ryan, Janna Ryan, Charlie Ryan

    Republican vice presidential candidate, Rep. Paul Ryan, R-Wis., his wife Janna and their son Charlie wave to supporters after Republican presidential candidate and former Massachusetts Gov. Mitt Romney conceded the race at his election night rally, Wednesday, Nov. 7, 2012, in Boston. (AP Photo/Stephan Savoia)