The United States has joined European human rights officials in expressing concern about the fairness of the Russian parliamentary elections.
President Putin faces election himself in March
Sunday's poll delivered a clear victory to allies of President Vladimir Putin.
But the Organisation for Security and Co-operation in Europe (OSCE) said the government used resources and control of the media to dominate the election.
The White House said Washington shared European concerns at how the United Russia Party achieved its success.
The election was "overwhelmingly distorted" by pro-government bias, the OSCE said.
With more than 98% of the votes counted, the pro-Kremlin United Russia Party is leading the poll with 37.1%.
The Communists took second place with 12.7%, followed by the ultra-nationalist party of Vladimir Zhirinovsky at 11.6%, according to preliminary results, Central Election Commission Chairman Alexander Veshnyakov told reporters.
Two liberal, pro-free market parties, the Union of Right Forces (SPS) and Yabloko, failed to get the 5% of votes needed to win party list seats in the State Duma, Mr Veshnyakov said.
The OSCE - which had about 400 observers in Russia - said the elections called into question Moscow's commitment to Western standards of democracy.
The organisation criticised the biased use of taxpayer money and state television to promote certain parties.
"In this election the enormous advantage of incumbency and access to state equipment, resources and buildings led to the election result being overwhelmingly distorted," said Bruce George, president of the parliamentary assembly of the OSCE.
In Washington, President George W Bush's spokesman referred to the importance of "Russian legislators dedicating themselves to pushing through the political and economic reform agenda".
But President Vladimir Putin has hailed the State Duma election as "another step in strengthening democracy" in Russia.
Experts say Mr Putin now seems assured of gaining a second term in presidential elections next March.
If United Russia and its allies are to gain a two-thirds majority, they would be able to amend the Russian constitution, potentially paving the way for Mr Putin to stand for a third term as president.
Twenty-three parties were competing for half of the 450 places in the State Duma of parliament, in the fourth such election since the collapse of communism in the early 1990s.
The other 225 seats were being contested by individual candidates on a first-past-the-post basis, where United Russia candidates were also expected to dominate, Russian experts say.
Election officials put voter turnout at about 56%, compared with 53.9% recorded during the last Duma vote in 1999.
The figure was well above the 25% mark needed to validate the poll.
Earlier, Communist leader Gennady Zyuganov denounced the elections as a "shameful farce".
Boost for Putin
The BBC's Stephen Dalziel says the slight polarisation in voting for the two second place parties does not necessarily spell trouble for Mr Putin.
Mr Zhirinovsky's misleadingly-named Liberal Democrats will be expected to support the president on most of his decisions as they did in his last parliament, he says.
But the result is being seen as a major defeat for the Communists, who have been a dominant force in the Duma for the past decade and were considered the greatest threat to the Kremlin.
Lyubov Sliska, a senior figure in United Russia, said the result meant democratic reforms would continue.
"This is a serious victory we can rightly be proud of," she said.
After nearly four years in the Kremlin, Mr Putin, the former head of the secret police, still appears to be riding a wave of genuine support.
The hard line his administration has taken against corruption and wealthy oligarchs has gone down well with many voters.