MOSCOW (AP) -- Thousands of Russians gathered Monday for a massive rally to challenge Prime Minister Vladimir Putin's victory in Russia's presidential election, chanting "Shame!" and "Russia without Putin!"
Demonstrators are contesting the outcome of the vote, pointing to a campaign heavily slanted in Putin's favor and to reports of widespread violations in Sunday's ballot.
Putin won more than 63 percent of the vote according to the nearly complete official returns, but the opposition and independent observers say the election has been marred by massive fraud.
"The campaign has been unfair, cowardly and treacherous," said opposition leader Grigory Yavlinsky, who had been denied registration for the race on a technicality.
International election monitors pointed at the lack of real competition and said the vote count "was assessed negatively" in almost a third of polling stations observers visited.
The violations in the vote added fuel to Monday's protest in downtown Moscow by Putin's foes, who are demanding an end to his 12-year rule. The rally, which follows a series of massive previous protests, has been sanctioned by authorities but security was tight, with some 12,000 police deployed to ensure order.
"We are going to hold new elections," said Yevgeny Natarov, a 38-year old Moscow resident who attended the opposition protest.
Sergei Udaltsov, one of the organizers, urged protesters to stay on Moscow's iconic Pushkin Square until Putin steps down.
"If it was a free election, why have they flooded the entire city of troops?" Udaltsov shouted to the crowd, which responded with cries: "They fear us!"
The police presence was intense near the site of Monday's protest and other areas of the capital, with dozens of trucks carrying riot shield-wielding police and vans used for detaining protesters parked around downtown. Prosecutors warned the rally's organizers they would face criminal responsibility for any unsactioned protests.
Police quickly rounded up Eduard Limonov, the leader of the banned National Bolshevik Party and several dozen of his supporters, who attempted to hold an unsanctioned protest near the headquarters of Russia's main security agency.
About 100 protesters were also arrested in St. Petersburg, where about 2,000 gathered for an unauthorized rally.
The independent Russian elections watchdog Golos said Monday that incomplete reports from its observers of individual polling station counts contradicted the official vote count, indicating that Putin hovered perilously close to the 50-percent mark needed for a first-round victory.
"It's one pixel away from a second round," said Golos' Roman Udot.
Putin's win was assured as he faced a weak slate of Kremlin-approved candidates and many across the vast country still see him as a guarantor of stability and the defender of a strong Russia against a hostile world, an image he has carefully cultivated during 12 years in power. He has denounced his foes as Western stooges working to weaken Russia.
Irina Titova contributed to this report from St. Petersburg.