Thousands protested against Prime Minister Vladimir Putin and his party Monday night in Moscow and St. Petersburg, saying their win in the parliamentary election was rigged.

Russian police officers detain an opposition activist during a protest against vote rigging in St. Petersburg on Sunday. (Dmitry Lovetsky/Associated Press)

Western observers have reported that the results in Sunday’s parliamentary election, in which United Russia took about 50 percent of the vote, were seriously distorted by ballot stuffing and other vote fraud. U.S. Secretary of State Hillary Clinton has called for a “full investigation of all credible reports of electoral fraud and manipulation.” While United Russia will continue to stay in power, the election results and ensuing protests indicate the first major threat to Putin’s party.

Below, see more images of the protests:

Police detain an activist during a protest rally by opposition group "Another Russia" in central Moscow Sunday. (Reuters)

Photos of the election:

View Photo Gallery: The United Russia party was predicted to get less than half of the vote even as election monitors and opposition groups reported widespread irregularities.

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