"The vote affirmed the main idea: that Vladimir Putin is the national leader, that the people support his course, and this course will continue," party leader and parliament speaker Boris Gryzlov said after exit polls were announced.

Several opposition leaders accused the Kremlin of rigging the vote, and U.S. President George W. Bush's administration called for a probe into voting irregularities. Communist Party leader Gennady Zyuganov called the election "the most irresponsible and dirty" in the post-Soviet era and party officials vowed to challenge the results.

With ballots from nearly 98 percent of precincts counted, United Russia was leading with 64.1 percent, while the Communists trailed with 11.6 percent, the Central Election Commission said.

Turnout was about 62 percent, up from 56 percent in the last parliamentary elections four years ago.

The Kremlin portrayed the election as a plebiscite on Putin's nearly eight years as president - with the promise that a major victory would allow him somehow to remain leader after his second term ends next year.

Putin is constitutionally prohibited from running for a third consecutive term, but he clearly wants to stay in power. A movement has sprung up in recent weeks to urge him to become a "national leader," though what duties and powers that would entail are unclear.

Pollsters said United Russia's performance would give it an overwhelming majority of 306 seats in the 450-seat State Duma, or lower house. The Communists would have 57 seats.

Two other pro-Kremlin parties - the nationalist Liberal Democratic Party and populist Just Russia - also appeared to have made it into parliament, with 8.2 percent and 7.6 percent, respectively.

One Liberal Democratic Party deputy will be Andrei Lugovoi, a former KGB officer and chief suspect in the poisoning death of Kremlin critic Alexander Litvinenko in London last year. Russia has refused to hand Lugovoi over to Britain, and the Duma seat provides him with immunity from prosecution.

No other parties passed the 7 percent threshold for gaining seats in the legislature. Both opposition liberal parties were shut out, predicted to win no more than 2 or 3 percent of the vote each.