Russia voted Sunday in legislative elections likely to erode the majority of Vladimir Putin's ruling party and marred by claims the authorities committed dirty tricks to ensure its dominance.

The United Russia party was expected again to win the majority in the State Duma parliament but with fewer seats than in the 2007 polls, 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 email was paralysed.

But it still was working via Skype and telephone, noting violations like the failure of the Volga city of Samara to allow observers inside to inspect ballot boxes before polls opened.

"Those people who wanted to stop us have not succeeded," said Arkady Lyubarev, the head of election monitoring at Golos.

Golos' chief had been held for 12 hours at a Moscow airport while the group said it was the target of a smear-documentary broadcast on pro-Kremlin Russian television. The United States said this pointed to "a pattern of harassment".

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.

Putin had lashed out at Golos last weekend, suggesting its activity was tantamount to betrayal of Russia. President Dmitry Medvedev 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 Moscow polling stations looking confident but without making comment.

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 percent 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.

Putin, who was recently subjected to unprecedented booing when he made an appearance at a martial arts fight, will want a strong performance from United Russia to consolidate his position as the country's paramount leader.

The main liberal opposition party taking part, Yabloko, was not expected to poll sufficient votes to enter parliament. Vehemently anti-Kremlin liberal party Parnas was not even allowed to take part and its leader Boris Nemtsov deliberately spoiled his ballot paper with the words "give us back our elections."

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 1500 GMT, almost 50.4 percent 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."

Police detained around 20 protesters including radical opposition leader Eduard Limonov for holding an unsanctioned protest in central Moscow, an AFP correspondent reported.