MOSCOW (AFP) -- Using Twitter and other social networks, some of the tens of thousands of volunteers monitoring Russia's polls Sunday posted evidence of alleged violations on the Internet and cast doubt on the legitimacy of the vote.

Election officials denied any significant fraud in the presidential polls set to be won by Vladimir Putin, which come after December 4 parliamentary elections were tarnished by claims of major vote-rigging.

A record number of volunteers signed up to monitor the casting and counting of ballots, with more than 27,000 monitors present at Russian polling stations, according to control2012.ru, an umbrella website coordinating their efforts.

The website listed more than 3,300 violations in the afternoon in Moscow, while Russian-language Twitter overflowed with evidence of doctored lists of voters and suspicious-looking buses near stations.

A series of videos made by a monitor in the Urals city of Yekaterinburg showed police detaining a minivan with about 10 people who admitted they had already voted several times.

"The carousel voters were detained and delivered to the police. We acted as witnesses," the monitor from the Communist party, Rostislav Zhuravlev, wrote on his Facebook.

Organised repeated voting -- also referred to in Russian as a "carousel" -- usually involves people using absentee voting documents to receive ballots at one polling station after another from corrupt officials.

At Moscow's central Bolotnaya Square, hundreds of buses were brought Sunday from other regions full of young people who said they came to vote for Putin, an AFP correspondent reported.

"I came here to vote for Vladimir Vladimirovich," said one young man from Belomorsk, a town in the Far North. "I voted for Putin," he said, without giving his name or saying why he had to come all the way to Moscow from his home town, over 1,000 kilometres (600 miles) away.

Blogging election monitors inundated Twitter with photos and licence plates of such buses bringing groups of people to stations in organised fashion.

   "Amazing. We were obviously waiting for carousels, but not to this extent," wrote protest movement leader Alexei Navalny. "Both (Moscow region governor Boris) Gromov and (Moscow mayor Sergei) Sobyanin should be sued for this."

"In Moscow and the Far East they ran out of absentee ballots despite the fact that extra copies were printed by the central election committee," said the head of the Golos monitoring group, Liliya Shibanova.

Liberal Yabloko party official Sergei Mitrokhin said a woman in the Siberian city of Omsk saw at the station's voters' list that her dead family members had come in to cast their ballots.

Some large companies even opened polling stations on their premises, where no cameras are present and where only the workers can vote.

"That is a very dangerous trend that worries us," said Golos electoral law expert Andrei Buzin. "It's hard to gain access to those stations, where people's voting is controlled. Before they did not open in such numbers."

Communist Party official Valery Rashkin said on the party's website that people in the Far East were bribed 1,000 rubles (over $30) to vote for Putin, and polling stations in Vladivostok launched a lottery offering voters a car and an apartment.

"The dirtiest elections of the past eight years have begun," he said.

But central election committee head Vladimir Churov dismissed allegations of foul play, saying in televised remarks that observers were overly "nervous" at the polling stations he visited, while "committees are working perfectly."