Russia's most popular social network has rejected a call by the country's domestic security agency to block opposition groups from using its website to organize post-election protests.

The network, VKontakte, said Thursday that the Federal Security Service asked it to deactivate the accounts of groups that posted calls for riots and a revolution. Spokesman Vladislav Tsyplukhin said VKontakte responded that it cannot stop the accounts of entire groups just because some individual users have called for violence. Tsyplukhin added that there was no pressure or threats from the security service.

Prime Minister Vladimir Putin's United Russia claimed victory in parliamentary elections Sunday, although with a reduced majority. But opposition groups and international observers claim that the vote was marred by violations, including ballot-stuffing, limited political competition and a lack of fairness.

Russian President Dmitry Medvedev Thursday acknowledged that Russia's electoral system was not perfect and said that accusations of fraud that led to large protests following Sunday's parliamentary elections will be investigated. But he said he believes the results reflect the political preferences of Russian society and that political observers agree the elections were well organized.

But Prime Minister Putin accused the United States of orchestrating the unrest in Russia, alleging that U.S. Secretary of State Hillary Clinton had encouraged his political opponents to take to the streets. Clinton said Tuesday the elections were neither free nor fair.

Opposition leaders oversaw three days of protests this week to express their frustration with the elections. Organizers have scheduled four more major protests to be held in Moscow over the next 10 days, with the first set for Saturday near the Kremlin. Authorities have granted a permit for 300 people, but 23,000 have pledged to attend through Facebook and the Russian social networking site.

Moscow has responded by ordering 50,000 police and 2,000 paramilitary troops backed by water cannon. Many protesters have been arrested.

Former Soviet leader Mikhail Gorbachev has called for Russia's election to be annulled and a new vote held. He said the results do not reflect the will of the people.