Vladimir Putin swept Sunday's presidential election to return to the Kremlin and extend his hold over Russia for six more years, incomplete returns showed.

His eyes brimming with tears, Putin defiantly proclaimed to a sea of supporters that they had triumphed over opponents intent on "destroying Russia's statehood and usurping power."

Putin's win was never in doubt as many across the vast country still see him as a guarantor of stability and the defender of a strong Russia against a hostile world, an image he has carefully cultivated during 12 years in power.

Accounts by independent observers of extensive vote-rigging, however, looked set to strengthen the resolve of opposition forces whose unprecedented protests in recent months have posed the first serious challenge to Putin's heavy-handed rule. Another huge demonstration was set for Monday evening in central Moscow.

With fewer than a quarter of the votes counted, Putin, the country's current prime minister who previously served as president between 2000 and 2008, spoke to tens of thousands of supporters at a rally just outside the Kremlin walls. Many of them were government workers or employees of state-owned companies who had been ordered to attend.

"I promised that we would win and we have won!" Putin shouted to the flag-waving crowd. "We have won in an open and honest struggle."

Putin, 59, said the election showed that "our people can easily distinguish a desire for renewal and revival from political provocations aimed at destroying Russia's statehood and usurping power."

He ended his speech with the triumphant declaration: "Glory to Russia!"

Exit polls cited by state television predicted Putin would get about 59 per cent of the vote. With more than 70 per cent of precincts counted nationwide, Putin was leading with 65 per cent, the Central Election Commission said. Complete results were expected Monday.

Communist Party candidate Gennady Zyuganov was a distant second, followed by Mikhail Prokhorov, the billionaire owner of the New Jersey Nets whose candidacy was approved by the Kremlin in what was seen as an effort to channel some of the protest sentiment. The clownish nationalist Vladimir Zhirinovsky and socialist Sergei Mironov trailed behind. The leader of the liberal opposition Yabloko party was barred from the race.

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Vladimir Putin, left, and his wife, Lyudmila, leave a polling station in Moscow on Sunday. Putin has claimed victory in the presidential vote. (Alexander Zemlianichenko/Associated Press)

The independent elections watchdog agency Golos said it was receiving reports of so-called "carousel voting," in which busloads of voters are driven around to cast ballots multiple times.

"There have been many people voting more than once, driven around in buses in large numbers" in Moscow, said Golos head Lilia Shibanova, who added similar reports had been received from Novosibirsk, Russia's third-largest city, and the city of Barnaul in southern Siberia.

Alexei Zafgrov, part of a group monitoring the election, told CBC News correspondent Karen Percy that there are many forms of voting fraud, but observers are motivated to stop it because they feel it's something they can do for their country.

"I don't think the end result of the election is a real big motivator …," Zafgrov said, adding that observers are trying to record information, such as videos and writing down licence plate numbers, to expose any violations.

"The goal of our effort is to actually get the true result, to make public the true result of the elections, because otherwise, if we allow this fraud to continue, no one will actually know how the public has voted."

'We must not relent'

Alexei Navalny, one of the opposition's most charismatic leaders, said observers trained by his organization also reported seeing extensive use of carousel voting.

"These are not going to be honest elections, but we must not relent," Mikhail Gorbachev, the last leader of the Soviet Union who has grown increasingly critical of Putin, said as he cast his ballot. "Honest elections should be our constant motto for years to come."

Evidence of widespread vote fraud in a December parliamentary election set off the protests against Putin, who has remained Russia's paramount leader after moving into the prime minister's office four years ago because of term limits. They were the largest public show of anger in post-Soviet Russia and demonstrated growing frustration with corruption and political ossification under Putin.

Some polling stations in Moscow that had been instructed to rig the vote in December were told to make sure Sunday's election was held "in full accordance with the law," an election official told The Associated Press on condition of anonymity.

He said the instructions were handed down at a meeting attended by the heads of about 50 polling stations.

The election official described how in December he had manipulated the vote at his polling station to give Putin's party the desired 65 per cent, when in fact it had won no more than 25 per cent.

At another Moscow polling station, where observer Kirill Raikov said he had witnessed a lot of ballot stuffing in December, the voting was orderly on Sunday.

"Compared to the previous election, everything here is calm and quiet," Raikov said. "We still cannot understand why this is happening."

The aim appears to be to take some of the steam out of the protest movement, which is centred in Moscow. 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 recognize vote-rigging, and record and report violations.

With files from CBC News