Fireworks were thrown at a protest rally in downtown Moscow Tuesday that pitted demonstrators denouncing alleged vote fraud in parliamentary elections against hundreds of pro-Kremlin youths.

Initially, there were reports that firebombs had been hurled but The Associated Press later said at least two flare-type fireworks thrown into a crowd of pro-Kremlin demonstrators gathered outside the Tchaikovsky Concert Hall on Tuesday night.

It was not immediately clear who had thrown them or whether they caused any injuries.

Police moved in quickly on the gathering.

Earlier, Russian police clashed with demonstrators trying to hold a second day of protests. Hundreds of police had blocked off Triumphal Square, then began chasing demonstrators, seizing some and throwing them harshly into police vehicles.

The Interfax news agency reported that among the detained was Boris Nemtsov, a leader of the liberal opposition. The demonstration followed a surprise Monday protest by thousands.

Moscow police spokesman Maxim Kolosvetov said about 250 people had been detained.

After Sunday's election, Prime Minister Vladimir Putin's United Russia party saw a significant drop in support but it will still maintain a majority in parliament. Opponents say even that watered-down victory was due to massive vote fraud.

Putin said he was satisfied with his party's performance in Russia's parliamentary election even though it lost a significant number of seats, saying that a drop in support was "inevitable" for any ruling party.

In neighboring Lithuania, U.S. Secretary of State Hillary Clinton again criticized the Russian election and urged that widespread reports of voting fraud be investigated.

Putin's United Russia party won about 50 per cent of Sunday's vote, a result that opposition politicians and election monitors said was inflated due to ballot-box stuffing and other vote fraud.

It was a significant drop from the 2007 election when United Russia took 64 per cent, gaining a two-thirds majority that allowed it to change the constitution.

Putin, however, said the ruling party had retained a "stable" majority.

"Yes, there were losses, but they were inevitable," he said.

"They are inevitable for any political force, particularly for the one which has been carrying the burden of responsibility for the situation in the country."

Vote violations denied

Russian officials have denied any significant vote violations.

The results reflected public fatigue with both Putin's authoritarian streak and widespread official corruption in Russia, signaling that his return to the presidency in next March's election may not be as trouble-free as he expected.

Anger against a heavy-handed state interference in the campaign in support of United Russia and evidence of vote fraud prompted thousands to march across downtown Moscow late Monday.

Police detained about 300 protesters in Moscow and 120 participants in a similar rally in St. Petersburg. One of the leaders, Ilya Yashin, who was among those arrested, was sentenced to 15 days in jail Tuesday for disobeying police.