Votes are being counted in the key battleground states that could decide whether Donald Trump or Hillary Clinton becomes the next occupant of the White House.
Polls have closed in Florida, which has been fiercely fought for by both sides in the bitter United States presidential race, while counting is also under way in North Carolina and Ohio.
Florida, which has 29 votes in the electoral college which decides who becomes the next president, would represent a major prize for either camp as they seek to reach the victory target of 270.
Ohio, which has 18 college votes, was won by Barack Obama for the Democrats in 2012, but Mr Trump's campaign has targeted the bellwether state, which has voted for the eventual occupant of the White House in every election since 1960.
Florida voted Democrat in 2012 and Mrs Clinton's hopes may hinge on a high turnout among Hispanic voters opposed to Mr Trump, who has courted controversy in the campaign with outspoken attacks on Mexican immigrants.
Elsewhere across the US, reports suggest that Democratic nominee Mrs Clinton has taken Vermont, as expected, while Mr Trump has won in Republican stronghold Indiana and in Kentucky.
Mr Trump has also taken Oklahoma, while Mrs Clinton won Delaware, Maryland, Massachusetts, New Jersey and the District of Columbia.
In an indication of Mr Trump's fraught relationship with the Republican establishment, former president George W Bush refused to vote for him, instead backing "none of the above".
In a sign that neither candidate has overwhelming support - even from their own voters - an early exit poll suggested only 42% said they "strongly favoured" the candidate they backed at the ballot box.
The CNN poll also indicated that the late surprises during the campaign - the renewed FBI interest in Mrs Clinton's use of a private email server while she was secretary of state and the emergence of a tape of Mr Trump making his "locker room" lewd comments about women - had not shifted voter intentions.
The poll suggested some 62% of voters decided before September which candidate they would back and only 12% decided over the past week.
The two presidential hopefuls are spending election night in New York, staging events barely more than a mile apart amid heavy security.
Mrs Clinton will address supporters at the Javits Centre in Manhattan, while Mr Trump has billed his speech at the Hilton Midtown hotel as a "victory party".
Mr Trump was booed as he arrived to vote at a school in Manhattan, accompanied by his wife Melania and daughter Ivanka.
His Democratic rival had earlier voted in her home town of Chappaqua, New York, along with her husband - and former president - Bill.
Mr Trump, who has claimed the election is "rigged" during his campaign, refused to declare if he would concede defeat if he loses on election day.
When asked in an interview with Fox News if he would accept the election result, he replied: "We're going to see how things play out."
Mrs Clinton is aiming to become the first female president in US history, while Mr Trump hopes his pledge to Make America Great Again will win over voters in key swing states.
Victory for Mrs Clinton would see her follow her husband into the White House, with the former president becoming the first gentleman - or "first laddie", as some have suggested.