President Vladimir Putin's party has won a landslide victory in nationwide elections, early official results show.
With 19% of ballots counted, United Russia had 63.5% of the vote, followed by the Communists at 11.3%.
Two other parties backing the Kremlin have also passed the 7% threshold to win seats. Turnout has been put at 60%.
This result would give Mr Putin the authority to retain political power after his presidential term ends next year, possibly as prime minister.
The Communists say they will mount a legal challenge to the result, citing alleged voting irregularities.
United Russia's leader, Boris Gryzlov, acknowledged there had been violations but dismissed them as insignificant.
"Of course there are violations but the question is do they have an impact on the final result... They in no way put in doubt the final result."
"The fact that these violations have been registered shows that we have a transparent ballot," he said.
Independent monitors have complained their attempts to observe the poll were hampered.
The Organization for Security and Co-operation in Europe (OSCE) abandoned its plans to send a big team of election observers, accusing Moscow of imposing curbs and delaying visas. Russia denied the claims.
Only a much smaller group of MPs from the OSCE's parliamentary assembly attended the election, leaving some 330 foreign monitors covering 95,000 polling stations.
Eleven parties are competing for places in the lower house, the Duma.
Several opposition parties have accused the government of stifling their campaigns.
Two parties allied to United Russia - A Fair Russia and the right-wing Liberal Democratic Party - also polled enough votes to enter parliament, according to early results.
Mr Gryzlov of United Russia said the election was "a referendum on President Putin so I think we can say he has won a victory".
The party will nominate its candidate for next year's presidential elections at a congress later this month, Mr Gryzlov said.
Mr Putin is constitutionally obliged to stand down after his second term as president ends in March next year.
According to the BBC 's James Rodgers in Moscow, his party's apparent landslide win in Sunday's election will enable him to continue wielding great influence in politics - even if he is no longer in high office.
Mr Putin announced earlier this year that he may consider trying for the office of prime minister after his presidential term ends.