MOSCOW (Reuters) - Russian riot police detained more than 500 protesters including opposition leader Alexei Navalny on Monday at rallies challenging the legitimacy of Vladimir Putin's victory in the presidential election.
Putin, who secured almost 64 percent of the votes on Sunday, portrayed his return to the presidency as a triumph over opponents who were trying to usurp power, though international monitors said the vote was clearly skewed in his favour.
But opposition leaders said they drew 20,000 people into Moscow's Pushkin Square, the scene of dissident protests during Soviet times, to call for new elections and an opening up of the political system crafted by Putin during his 12-year rule.
"They robbed us," Navalny, a 35-year-old anti-corruption blogger, told the crowd before his detention. "We are the power," he said to chants of "Russia without Putin" and "Putin is a thief."
The atmosphere at the rally was jovial at first, but became tense when riot police in helmets moved in to disperse several thousand activists who stayed on the square.
Encircling one group of protest leaders huddled in a fountain closed down for winter, black-helmeted riot police detained Navalny and others and marched them to waiting police vans.
Opposition leaders said 500-1,000 people were detained but police put the number at 250 and said 14,000 people had attended the rally.
The U.S. ambassador to Russia, Michael McFaul, said on Twitter that the arrests were troubling and that freedom of assembly and speech were universal values.
Thousands of Putin supporters staged rallies closer to the red walls of the Kremlin, singing songs, waving Russian flags and chanting Prime Minister Putin's name.
At least 300 people were detained by riot police at unsanctioned protests in the northern city of St Petersburg, Putin's home town, a police spokesman said. Up to 3,000 people turned out in St Petersburg, witnesses said.
Putin says he won a six-year term as Kremlin chief in a fair and open contest, but vote monitors from the Organization for Security and Co-operation in Europe echoed the opposition's complaints that the election had been slanted to help him.
"The point of elections is that the outcome should be uncertain. This was not the case in Russia," Tonino Picula, one of the vote monitors, said on Monday. "According to our assessment, these elections were unfair."
The U.S. State Department called for an "independent, credible" investigation into all reported violations.
The monitors said there had been some improvements from a parliamentary poll on December 4 which observers said was marred by irregularities, but said Putin still had an advantage over his rivals through massive media coverage and the use of state resources to help him extend his domination of Russia for six more years.
Although the observers' findings have no legal bearing, they undermine Russian election officials' statements that there were no serious violations.
They would also support some in their view that elections ultimately have little real significance in Russia; that power is tightly controlled and divided up by a largely stable ruling clique, as demonstrated by the 'tandem' power deal struck by Putin and current President Dmitry Medvedev in 2008.
Putin's opponents, fearing he will smother political and economic reforms, have refused to recognise the result, which could allow the former KGB spy to rule Russia for as long as Soviet leader Leonid Brezhnev, accused of presiding over "the years of stagnation."
"I used to love Putin, like any woman who likes a charismatic man. But now I think he is getting senile. Nobody can stay in power forever," Vasilisa Maslova, 35, who works in the fashion trade, said during the opposition rally.
"Voting yesterday, I felt like I was choosing the least dirty toilet in a crowded train station."
Protesters in Moscow held up banners reading "12 more years - no thanks" and "We need a Russian president, not a leader for life."
In a conciliatory move, Putin invited his defeated presidential rivals to talks, although Communist Party candidate Gennady Zyuganov did not attend.
The Kremlin also took steps that appeared intended to try to take the sting out of the protests which began over the December 4 poll won by Putin's United Russia party.
Medvedev, who will stay in office until early May, told the prosecutor general to study the legality of 32 criminal cases including the jailing of former oil tycoon Mikhail Khodorkovsky.
Khodorkovsky, who headed what was Russia's biggest oil company, Yukos, and was once the country's richest man, was arrested in 2003 and jailed on tax evasion and fraud charges after showing political ambitions and falling out with Putin.
The Kremlin said Medvedev had also told the justice minister to explain why Russia had refused to register a liberal opposition group, PARNAS, which has been barred from elections.
The order followed a meeting last month at which opposition leaders handed Medvedev a list of people they regard as political prisoners and called for political reforms.
Medvedev's initiatives "have only one goal: To at least somehow lower the scale of dismay and protest that continues to surge in society," Zyuganov said.