Russians have voted in polls to decide the shape of the lower house, or Duma, for the next five years.
A first exit poll indicated 48.5% support for the United Russia party of Prime Minister Vladimir Putin, down from 64% in 2007.
That would give United Russia 220 seats in the new Duma, down from 315.
Russia's only independent monitoring group, Golos, logged 5,300 complaints alleging violations of election laws and said its website had been hacked.
The Golos monitors, who are not affiliated with any party, are funded largely by the US and EU.
Prime Minister Vladimir Putin has accused foreign powers of meddling in election preparations.
Duma members have questioned why the foreign-funded organisation - whose name means "voice" or "vote" - is allowed to monitor Russian elections.
Moscow police said more than 100 people were arrested at an opposition demonstration.
Long voting day
The previous Duma was widely viewed as little more than a rubber stamp for the Kremlin, says the BBC's Steve Rosenberg in Moscow - adding that this may explain why the campaign has failed to excite the Russian public.
The election is being seen as a referendum on Mr Putin's personal popularity, three months before the Russian prime minister runs again for president. He served two terms in the post between 2000 and 2008.
Voting for representation in the country's 450-seat lower chamber takes place across Russia's nine time zones with polls closing at 17:00GMT on Sunday in Kaliningrad.
As people in European Russia were going to bed, polls were opening in the far east.
In Vladivostok, sailors from Russia's Pacific fleet were among those queuing to vote.
Warrant Officer Nikolai Ponomaryov said he was voting for United Russia, because the party was supporting the armed forces.
In the far north, above the Arctic circle, voters braved temperatures of -26C to cast their ballots.
Polling stations opened in the capital Moscow early on Sunday.
On Friday, Golos was fined the equivalent of $1,000 (£641) by a Moscow court for violating a law that prohibits publication of election opinion research for five days before a vote.
Many of the allegations about electoral malpractice involved Mr Putin's United Russia.
On Sunday, Golos said its website, which lists alleged abuses, had been targeted by a "denial-of-service" cyber-attacks.
A number of media critical of the Kremlin, including the popular radio station Moscow Echo, also said their sites were unavailable.
"The attack on the website on election day is clearly an attempt to inhibit publication of information about violations," Moscow Echo editor Alexei Venediktov wrote on Twitter.
Earlier, Golos head Liliya Shibanova said officials had taken her laptop as she arrived at Moscow airport on Saturday, on the pretext that it had illegal software.
"This is a provocation directed personally against me," she said, alleging that the authorities were trying to prevent her from travelling to the European parliament next week.
Seven parties have been allowed to field candidates for parliament this year - down from 11 in 2007.
The outgoing Duma is dominated by Mr Putin's party, with seats also held by the Communist Party, the nationalist Liberal Democrats and the social-democratic Fair Russia.
In a televised address on Friday, President Dmitry Medvedev insisted Russia's political parties enjoyed "free and equal competition" ahead of the election.
Without naming United Russia, he urged voters to choose "responsible politicians, who can help improve our people's living standards in practice, and who will be guided in their actions by the interests of voters and national interests".