Russia voted on Sunday in legislative elections set to see an erosion in the majority of Vladimir Putin's ruling party and marred by claims the authorities have engaged in foul play to ensure its dominance.

The United Russia party of Putin was expected again to win the majority in the State Duma parliament but with fewer seats, as opponents said the ruling elite had downed websites and harassed monitors to limit dissent.

The elections are seen as a crucial test of Putin's popularity in Russia as he prepares to stand in March 2012 presidential elections to return to the Kremlin after his four-year stint as prime minister.

Independent monitor group Golos (Voice), which claimed rampant violations in the campaign, said its Map of Violations website documenting reports of fraud was inaccessible due a cyber-attack and its communications had been paralysed.

"Our email is not working, and we only have Skype to communicate with our regional network," spokeswoman Olga Novosad said.

Meanwhile, the website of popular radio station Moscow Echo, which is owned by state gas monopoly Gazprom but often tackles sensitive issues, was the subject of a similar distributed denial of service (DDoS) attack.

"The attack on the website on election day is clearly an attempt to inhibit the publication of information about violations," Moscow Echo editor-in-chief Alexei Venediktov wrote on Twitter.

On Friday, Golos was fined nearly $1,000 and was the subject of a prime-time television programme that accused the 'ostensibly independent observers' of acting in the interests of the US government.

Customs officials held Golos head Lilia Shibanova for 12 hours at Moscow's Sheremetyevo airport and confiscated her laptop on Saturday.

US President Barack Obama's administration was alarmed by 'a pattern of harassment directed against this organisation (Golos)', US National Security Council spokesman Tommy Vietor said in a statement.

Putin had lashed out at Golos last weekend, comparing it to Judas. President Dmitry Medvedev on Friday rejected claims of foul play saying elections were 'one of the greatest manifestations of democracy'.

Russia's two rulers - who are set to swap jobs in 2012 with Medvedev becoming prime minister - voted at separate polling stations.

The four years since the last parliamentary vote in 2007 have been marked by an outburst of criticism of the authorities on the Internet as web penetration of Russia started to finally catch up with the rest of Europe.

Opinion polls have predicted that United Russia's nationwide poll rating will drop from 2007 when it secured a landslide majority of 64.3 per cent and won 315 seats in the State Duma.

The three main opposition parties - the Communists, the nationalist Liberal Democrats and the populist A Just Russia - should all see their support tick up without posing any significant challenge to United Russia.

The marathon election process in the world's largest country kicked off in Pacific Ocean regions and was to conclude 21 hours later with the close of polls in the exclave of Kaliningrad bordering the European Union, nine time zones away.

Turnout will also be closely watched to see how many Russians are disillusioned with the political process after more than a decade of Putin's strongman rule.

According to information received by 0900 GMT, over 25 per cent of the electorate had cast their vote, the election commission said.

Some Russians said they would support Putin's United Russia, while others noted they had so far seen nothing from it but empty promises.

Nikolai Ponomaryov, a warrant officer from the Marshal Shaposhnikov anti-submarine warship based in the Pacific port of Vladivostok, said he voted for Putin's party because he saw changes for the better.

"Already this spring my family will get an apartment in a new district," he said.

"I link these changes with the work of United Russia."

But Stanislav Dvorkovoi, 35, a voter in Moscow, said that he was voting from the Communists 'so that the State Duma does not have a monopoly of one party'.

Putin, who was recently subjected to unprecedented booing when he made an appearance at a martial arts fight, and Medvedev have made clear they did not want to see a squabbling parliament like that in the 1990s under Boris Yeltsin.

Pro-Kremlin youth group Nashi (Ours) said that 15,000 of its members would be moving around Moscow during the vote while the members of the radical opposition said they would stage unsanctioned protests later in the day.