According to the preliminary data, the Communist Party received 11.4%, the ultranationalist Liberal Democratic Party 9.5%, and A Just Russia, led by Kremlin loyalists, garnered 7.5%.
No other parties made it past the 7% threshold needed to enter parliament. The Agrarian party received 2.5% of votes, Yabloko 1.4%, and Union of Right Forces (SPS) 1.1%.
The Central Election Commission put voter turnout at around 60%, compared to 55.75% at the 2003 State Duma elections.
Speaking after the partial vote count was announced, the head of United Russia said the party would not use its overwhelming parliamentary majority to force changes to the Constitution.
Boris Gryzlov, who is also the Duma speaker, said that over the past four years the party has used its parliamentary majority to "protect the current Constitution."
He called the result a victory for President Vladimir Putin, who topped the party list.
Gryzlov conceded that there were violations in the State Duma elections, but said that these did not affect the result.
Voting finished at 9:00 p.m. Moscow time (6:00 p.m. GMT), when the last polling stations closed in the westernmost part of Russia, the Baltic exclave of Kaliningrad.
The parliamentary elections were the fifth in Russia's post-Soviet history.
The run-up to the Duma elections was marred by a dispute with the Organization for Security and Cooperation in Europe, after its main election monitoring arm refused to dispatch observers for the polls, citing restrictions and visa delays. Moscow in turn criticized "chaos" in the organization and reinvigorated calls for its reform.
Russia's Communist Party said on Sunday it is preparing lawsuits to contest the results of the elections.
"A group of our lawyers have already begun preparing lawsuits for the Supreme Court to contest the results of voting, without waiting for the vote count," said the head of the party's legal service, Vadim Solovyev.
"The barrage of violations exceeds all acceptable norms," he added.