UPDATED at 6:18 p.m. ET: Thousands of protesters chanting "Russia without Putin" took to the streets of Moscow and St. Petersburg on Monday to challenge Vladimir Putin's victory in a presidential election that international monitors said was unfair.
Russian riot police detained more than 500 protesters including opposition leader Alexei Navalny at rallies challenging the legitimacy of Putin's victory.
Putin, who secured almost 64 percent of votes on Sunday, portrayed his return for a third term as president as a strong mandate to deal with the protests.
'Serious problems' with vote that kept Vladimir Putin in power, monitors say
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.
Lines of officers in full riot gear marched into tree-lined Pushkin Square and forced protesters into waiting police buses. About 250 people were detained around the city, police said.
The opposition remained defiant.
"Yesterday was not a vote. Yesterday was a falsification," Boris Nemtsov, a liberal opposition leader, told the rally in Moscow's Pushkin Square, where Soviet-era dissidents used to stage protests.
The crowd, waving flags and anti-Putin banners, roared back: "No!"
"They fear us ... but we do not fear these monsters," Nemtsov said.
PhotoBlog: At least 100 anti-Putin activists arrested in Russia
A Reuters reporter saw several people being manhandled as police took them away when they protested near the former headquarters of the KGB security police, where the police said they had made 50 arrests.
Police detained at least 50 people at an unsanctioned rally of about 3,000 people in St. Petersburg, witnesses said.
Meanwhile, hundreds of Putin supporters staged rallies closer to the red walls of the Kremlin, singing songs, waving Russian flags and chanting the president-elect's name.
'Unfair' vote
Before the protests, vote monitors from the Organization for Security and Co-operation in Europe echoed the opposition's complaints that the election was slanted to favor Putin.
"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."
French Foreign Minister Alain Juppe said the election had not been exemplary "to say the least."
Tiny Cox, one of the most senior electoral monitors, said there had been some improvements from a parliamentary poll which observers said was marred by irregularities on Dec. 4.
"We did not see the violations we saw in December. We saw far less cases of ballot-box stuffing," he said.
But the OSCE monitors said Putin still had an advantage over his rivals in the media and that state resources were used to help him extend his domination of Russia for six more years.
Anti-Putin activists pay high price, but refuse to back down
Expressing concerns that a European Union spokeswoman said were shared by the 27-country bloc, the monitors called for all allegations of irregularities to be thoroughly investigated.
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 something 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.
"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."
Putin's opponents, fearing he will smother political and economic reforms, have refused to recognize 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."
Putin, 59, has already served as president or premier for 12 consecutive years and made way for his ally Medvedev in 2008 because of constitutional limits.
"He (Putin) is forcing things to breaking point. He is declaring war on us," said journalist Sergei Parkhomenko, one of the protest organizers.
Many voters still see Putin as a safe pair of hands and credit him with restoring order after the chaotic 1990s under President Boris Yeltsin and overseeing an economic boom.
But others have lost all faith in elections and see Putin as an impediment to Russia having a fair, decent society.
"We've had enough lies. The whole country has had enough lies," said Rosa Trukachova, a 60-year old pensioner.
The Obama administration on Monday congratulated the Russian people for turning out to vote in big numbers in this weekend's presidential election but said it was concerned by allegations of massive fraud and pointedly did not mention victor Vladimir Putin by name.
In a statement, the State Department said the U.S. would work with Russia's "president-elect" once the vote is certified. It noted that European observers had determined that "the election had a clear winner with an absolute majority." At the same time, it pointed out that the monitors had raised issues with the fairness of the campaign, partisan use of government resources and procedural irregularities on election day. It said those charges must be fully and credibly investigated.
The Associated Press and Reuters contributed to this report.
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