MOSCOW — Thousands of protesters took to the streets of Moscow on Monday to voice their anger at polling irregularities they fear will allow the ruling party to maintain control of the parliament’s lower house, despite a relatively weak showing in elections held Sunday.

Accusations of ballot stuffing and voter fraud were voiced by the demonstrators as well as international observers, on a day when election officials said preliminary results could give the United Russia party of Prime Minister Vladimir Putin and President Dmitry Medvedev 238 seats in the 450-seat State Duma, with more than 49 percent of the vote. In 2007, the party won 64 percent of the vote.

“A thief breaks in our home and tells us to go on watching television while he is robbing us out of our possessions!” said liberal blogger Alexei Navalny, addressing a crowd of mostly young people.

Secretary of State Hillary Clinton said Monday in Bonn, where she was attending a conference on Afghanistan, that “Russian voters deserve a full investigation of all credible reports of electoral fraud and manipulation.”

Medvedev, at a meeting of supporters, said the election was fair.

“The United Russia got exactly what it has — not less, not more — and in this sense it was an absolutely honest and fair democratic election,” he said.

“All this talk about the boundless use of administrative resource. … Where is this resource?”

The Communist Party was trailing in the preliminary results with more than 19 percent, followed by Just Russia, a splinter wing of the ruling party that rebelled against the Kremlin, with more than 13 percent, and the nationalist Liberal Democratic party with more than 11 percent. The other three parties on the ballot, including the liberal Yabloko party, will not receive parliament seats because they didn’t pass the 7 percent threshold.

Accusations of ballot fraud by United Russia, which controls an absolute majority of regional and local administrations, mounted even before Sunday’s voting concluded.

Programmer Vasily Dovedov, a Communist party election observer, said at the protest that he “personally caught by the hand a man at polling station No. 2829 (in a northwestern Moscow suburb) in the act of stuffing into a ballot box more than a dozen ballot papers all marked in favor of United Russia.”

Fellow election observer Alexandra Richard said that another polling station had issued 1,124 ballot papers, but that 1,505 ballots were in the boxes when the count took place.

“How can the United Russia get 46 percent in Moscow when the exit polls wouldn’t give them more than 22 percent?” asked the 21-year-old law student. “It is clear that they cheated and we have proof of that.”

“Putin out! Putin out!” chanted the protesters Monday night. “We want new elections!”