The United Russia party of President Vladimir Putin has secured more than 60% of the vote in Sunday's election, preliminary official results indicate.
Mr Putin's party said the result showed he was "national leader"
Only two other pro-Kremlin parties and the opposition Communists appear to have got sufficient votes to pass the threshold needed for seats in the Duma.
United Russia said the result showed Russians supported Mr Putin's course.
The US has urged Moscow to investigate claims of voting irregularities by opposition leaders and the OSCE.
United Russia has admitted there were some irregularities on Sunday, but said they were not significant enough to affect the result.
Mr Putin has said a strong result would give him the authority to retain political power after his presidential term ends next year, possibly as prime minister.
With nearly 98% of ballots counted, the Central Election Commission announced on Monday morning that United Russia was leading with 64.1%, while the opposition Communists trailed with 11.6%.
Only two other parties - the Liberal Democratic Party of Russia and A Fair Russia, which are allied to United Russia - are on track to clear the 7% threshold required to qualify for seats.
The result would mean pro-Kremlin parties would have the two-thirds majority in the Duma required to able to amend the constitution.
Yabloko, the standard-bearer of the liberal left in the country, was a distant sixth with only 1.6% of the vote.
The election commission said the turnout amongst Russia's 109m voters had been 63%, up from 56% in the last parliamentary election four years ago.
United Russia's leader, parliamentary speaker Boris Gryzlov, hailed the result as a signal from Russian voters that they wanted President Putin to retain influence and his policies to be maintained even after he leaves office in 2008.
"The vote affirmed the main idea - that Vladimir Putin is the national leader, that the people support his course, and this course will continue," he said after the results of exit polls were announced on Sunday.
Kremlin spokesman Dmitry Peskov said the result was "extremely important from the point of view of continuation... of the reforms started in the last eight years".
Mr Putin is constitutionally obliged to stand down after his second term as president ends in March next year.
Mr Peskov said Mr Putin planned to "transfer his presidency to another man" following the presidential election in March, while United Russia announced it would nominate its candidate for the post at a party congress on 17 December.
The BBC's James Rodgers in Moscow says United Russia's landslide win on Sunday will enable Mr Putin to continue wielding great influence in politics - even if he is no longer in high office.
However opponents have been crying foul.
The Communists and several smaller opposition parties have said they will mount a legal challenge in the Supreme Court .
Communist leader Gennady Zyuganov said the election had been the "most irresponsible and dirty" in the post-Soviet era and warned that the new parliament would be a "subsidiary of the Kremlin, a rubber-stamp factory".
The election also came under strong criticism from the Organisation for Security and Co-operation in Europe (OSCE).
Reports of fraud
"The executive branch acted as though it practically elected the parliament," the OSCE's Kimmo Kiljunen told the Moscow Echo radio station.
The OSCE abandoned its plans to send a big team of election observers, accusing Moscow of imposing curbs and delaying visas. Russia denied the claims.
The independent Russian monitoring group Golos said violations had amounted to "an organised campaign".
Turnout at the election is believed to have been very high
Spokeswoman Tatyana Bogdanova said observers had reported cases where state employees and students had been pressured to vote.
The White House urged Russian authorities to investigate the reports of fraud.
However, the chairman of the Central Election Commission, Vladimir Churov, told Russian television he knew of "no serious violations in the course of polling day".
Mr Gryzlov also acknowledged there had been violations but dismissed them as insignificant.
The election also appears to have delivered a parliamentary seat to Andrei Lugovoi, the man the UK wants to put on trial for the murder of former Russian spy Alexander Litvinenko.
He has denied the charge, but status as a member of parliament for the Liberal Democratic Party would bring with it immunity from prosecution in Russia.