Russians have voted across nine time zones in parliamentary elections, as opposition groups continue to report poll violations.
Voting began at 2000GMT on Saturday in Russia's far eastern regions and is due to finish 21 hours later at 1700GMT on Sunday in the enclave of Kaliningrad, wedged between Poland and Lithuania.
Meanwhile, an electoral watchdog has complained of "massive cyber attacks" on a website alleging violations.
Campaigning ended on Friday but election posters were allowed on Saturday far away from polling stations if they had been posted earlier.
Moscow neighbourhoods were dotted with United Russia posters, but many of the other parties' posters had been removed.
Al Jazeera's Neave Barker, reporting from Moscow, said: "The country's only independent election observer called Golos reported that more than 5,000 irregularities have been recorded, many of them connected with people pressured to vote mainly for the country's biggest and most powerful party, United Russia".
"There have been some skirmishes today in and around Red Square," our correspondent said. "The capital is on lockdown, and the police are looking for any sign of trouble, with more protests expected later on."
'Theatre of the absurd'
Speaking to Al Jazeera, Garry Kasparov, a political activist and former world chess champion, said: "All the other parties participating in this so called election are 100 per cent under the Kremlin's control.
"Voting for them is to vote for puppets in the theatre of the absurd."
Russian bloggers said Livejournal, their most popular website, was down for the third consecutive day, with some alleging a cyberattack by the authorities to prevent people from discussing the vote.
Media also reported that the website of Golos, a Western-funded poll monitoring group, had also been down. The group had published a map documenting where violations were alleged to have taken place.
The United Russia party, which is chaired by Vladimir Putin, Russia's prime minister, is expected to win the poll, both he and Medvedev have struggled during a lacklustre election campaign to prevent its huge majority being cut, after signs of weariness with the party.
Yet Putin remains by far the most popular politician in the vast country of more than 140 million people but there are some signs Russians are tiring of his cultivated strong-man image.
The 59-year-old ex-spy looked stern and said only that he hoped for good results for his ruling United Russia party as he
walked past supporters before voting in Moscow.
Ruling party leaders said it would be a step backwards if voters chose a Duma, the lower house of parliament, as divided as it was during the 1990s, when rival parties battled for supremacy. United Russia has dominated since 2003.
If United Russia does not retain its two-thirds majority, the biggest gainers would likely be the Communist Party - the second-biggest force. Vladimir Zhirinovsky's nationalist LDPR, the third-largest party, also hopes to gain votes.
Poll monitor detained
Russian customs officers held the director of Golos for 12 hours at a Moscow airport on Saturday, seizing her laptop computer in what the group said was an attempt to stop it monitoring the election.
Liliya Shibanova, executive director of Golos, was detained at Sheremetyevo airport after returning from abroad for the election.
|Members of United Russia endured rare public
heckling leading up to Sunday's vote [Al Jazeera]
A Moscow court ruled late on Friday that Golos had violated a ban on the publication of opinion poll results within five days of the election to the Duma.
Golos and its lawyers said they would appeal against the court decision, which came with a $970 fine. They said Golos had published allegations of campaign violations, not opinion poll results.
Russian MPs had earlier objected to Golos' foreign financing and urged it to stop vote monitoring.
Golos has a hotline for electoral violation allegations and an interactive map showing more than 8,000 reported violations linked to Sunday's vote.
It acknowledges funding from Europe and the US, saying this helps it remain objective.
The White House on Friday condemned Moscow's "pattern of harassment" against Golos following the fine.
"The Obama administration is concerned with today's decision by a Moscow court regarding the election monitoring NGO Golos, as well as what appears to be a pattern of harassment directed against this organisation," Tommy Vietor, the national security council spokesman, said in a statement.
He said Washington had shared concerns with officials both in Moscow and at the Russian embassy in Washington.