Thousands of Russians gathered for a massive rally Monday in downtown Moscow to challenge Prime Minister Vladimir Putin's victory in Russia's presidential election.

Security at the event was tight with more than 12,000 police officers in place to ensure the event was orderly.

The demonstrators were contesting the outcome of Sunday's vote, pointing to a campaign heavily slanted in Putin's favor and to reports of widespread election violations.

Putin won more than 63 percent of Sunday's vote, according to the nearly complete official returns.

"The campaign has been unfair, cowardly and treacherous," opposition leader Grigory Yavlinsky said. Yavlinsky had been denied registration for the race on a technicality.

International election observers pointed at the lack of real competition and said the vote count "was assessed negatively" in almost a third of polling stations they visited.

Putin has been the head of the Russian government for the past 12 years.