Prime minister Vladimir Putin has declared victory in Russia's presidential election as exit polls and preliminary results gave him around 60% of the vote.
Speaking at a rally outside the Kremlin, Mr Putin thanked his supporters ahead of a concert arranged to celebrate his victory.
With tears in his eyes, he said: "I promised you we would win, and we won. Glory to Russia!"
"We have won in an open and honest battle. We proved that no-one can force anything on us."
The vote was tainted by claims of violations, including "carousel voting" in which voters were bused around to cast several ballots.
Mr Putin tallied 58%, according to a nationwide exit poll conducted by the VTsIOM polling agency. Another exit poll by the FOM opinion survey service showed Mr Putin received 59% of the ballot.
Official vote results from the far eastern regions where the count was already completed seemed to confirm the poll data. With just over 20% of all precincts counted, Mr Putin was leading the field with 63% of the vote, the Central Election Commission said.
But if thousands of claims of violations made by independent observers and Mr Putin's foes are confirmed, they could undermine the legitimacy of his victory and fuel protests. The opposition is gearing up for a massive rally in central Moscow on Monday.
"These elections are not free ... that's why we'll have protests tomorrow. We will not recognise the president as legitimate," said Mikhail Kasyanov, who was Mr Putin's first prime minister before going into opposition.
Golos, Russia's leading independent elections watchdog, said it received numerous reports of "carousel voting", in which busloads of voters are driven around to cast ballots multiple times. Alexei Navalny, one of the opposition's most charismatic leaders, said observers trained by his organisation also reported seeing extensive use of the practice.
Evidence of widespread vote fraud in December's parliamentary election drew tens of thousands to protest against Mr Putin, who was president in 2000-2008 before moving into the prime minister's office due to term limits.
They were the largest outburst of public anger in post-Soviet Russia and demonstrated growing exasperation with massive corruption, rising social inequality and tight controls over political life under Mr Putin.
Mr Putin has dismissed the protesters' demands, casting them as a minority of urban elites working at Western behest to weaken Russia. His claims that the United States was behind the opposition protests resonated with his core support base of blue-collar workers, farmers and state employees, who are suspicious of Western intentions after years of state propaganda.
The Communist Party candidate, Gennady Zyuganov, was trailing far behind Putin with 18%, according to the exit polls. The others - nationalist Vladimir Zhirinovsky, Sergei Mironov of the socialist Just Russia party and billionaire Mikhail Prokhorov - were in single digits.
Mr Putin has promised that the vote would be fair, and the authorities have apparently sought to take the steam out of the protest movement by allowing more observers to monitor the vote.
Tens of thousands of Russians, most of them politically active for the first time, had volunteered to be election observers, receiving training on how to recognise vote-rigging and record and report violations.
On Sunday David Miliband described Mr Putin as a "ruthless" dictator whose days are numbered, as Russia went to the polls.
The former foreign secretary warned it would be wrong to underestimate the "intelligent" leader but predicted he will not survive a six-year term at the Kremlin.
In an article for The Sun On Sunday, he wrote: "Whether or not Vladimir Putin wins today, he will not be celebrating a fourth term in office six years from now.
"Whoever wins the election today, one thing is clear: Russia will not be the same.
"The people of Russia have spoken up, and a wise leader would listen."
Mr Miliband wrote: "Russian nationalists, communists and liberals who have taken to the streets in their tens of thousands in temperatures well below freezing don't agree about much, but they are united against corruption, stagnation and arbitrary rule.
"It is wrong to underestimate Putin. He is intelligent, worldly and ruthless. In the first term of his presidency, in the wake of the embarrassing latter years of Boris Yeltsin, the rhetoric and to some extent reality was about reform as well as order.
"Russians got their pride back - floating on a tide of oil and gas revenues. But since then Russian reform has gone into reverse, and vested interests consolidated their positions."
Relations between Britain and Russia reached breaking point after the murder in London of dissident critic Alexander Litvinenko.