Updated at 1:08 p.m. ET
(AP) MOSCOW - Thousands of Russians rallied Monday to challenge Prime Minister Vladimir Putin's victory in Russia's presidential election, crowding into a central Moscow square to chant "Shame!" and "Russia without Putin!"
Trying to sustain the momentum of three months of unprecedented demonstrations against his heavy-handed 12-year rule, the opposition is pointing to a campaign slanted in Putin's favor and reports of widespread violations in Sunday's ballot.
Organizers said about 20,000 people showed up, far short of the crowds of 100,000 that turned out for previous rallies but protesters were squeezed into a far smaller space on the iconic Pushkin Square and it was too early to gauge how intense the outrage would be.
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.
"There was no real competition, and abuse of government resources ensured that the ultimate winner of the election was never in doubt," said Tonino Picula, the head of the short-term Organization for Security and Cooperation in Europe observer mission. "Broadcast media was clearly biased in favor of one candidate and did not provide fair coverage of the other candidates."
Russian observers pointed at numerous reports of "carousel voting," in which busloads of voters are driven around to cast ballots multiple times and various other violations, saying their number appeared to be as high as in December's disputed parliamentary vote that kicked off the protests.
Monday's rally had been sanctioned by authorities but security was tight, with some 12,000 police deployed to ensure order. The big rally went on peacefully, but police detained about more than a dozen of people who tried to walk toward the Kremlin after the protest was over.
A few hundred demonstrators remained on the square after the rally was over, heeding protest leaders call to stay and raise the heat on authorities. Hundreds of riot police surrounded them, but didn't make any immediate attempt to disperse the protest.
"We are going to hold new elections," said Yevgeny Natarov, a 38-year old Moscow resident who attended the opposition protest.
"I have nothing to fear," said Vladimir Belyayev, a 62 year-old protester who held a placard reading "People, where is your self-dignity?"
Sergei Udaltsov, one of the organizers, urged protesters to stay on the square until Putin steps down.
"If it was a free election, why have they flooded the entire city with troops?" Udaltsov shouted to the crowd, which responded with cries: "They fear us!"
After the rally ended, Udaltsov was joined by opposition lawmaker Ilya Ponomaryov and several hundred protesters who stayed on the square, chanting "We shall not leave!"
The police presence was intense, 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 unsanctioned 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 a 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 relied on massive coverage by state television stations, denouncing his foes as Western stooges working to weaken Russia.
Putin claimed victory Sunday night when fewer than a quarter of the votes had been counted, his eyes brimming with tears. He defiantly proclaimed just outside the Kremlin walls before a sea of supporters that they had triumphed over opponents intent on "destroying Russia's statehood and usurping power."
U.S. Sen. John McCain, who had goaded Putin in the past on Twitter, reacted quickly to the images ofwith an acerbic Tweet:
"We have seen a man who wasn't sure of himself," said Ilya Yashin, one of the opposition leaders.
Mikhail Kasyanov, who served as prime minister during Putin's first term before turning an opposition leader, urged the protesters to focus on demanding a rerun of the fraud-tainted parliamentary election in December, which allowed Putin's party to retain its majority in the lower house.
"Early Duma election is our immediate goal!" he shouted. "Putin is afraid of us!"
In an apparent bid to assuage the opposition anger, outgoing President Dmitry Medvedev told the Justice Ministry to present its explanation for last year's rejection of registration for the People's Freedom Party, an organization led by some of the opposition's most prominent figures.
He also ordered the prosecutor-general to re-examine the legality of the conviction of imprisoned former oil tycoon Mikhail Khodorkovsky and more than 30 others regarded by the opposition as political prisoners.
Political analysts and opposition activists saw the move as an attempt to soothe protesters, but said that it remains to be seen if it results in any action.
The West can expect Putin to continue the tough policies he has pursued even as prime minister, including opposing U.S. plans to build a missile shield in Europe and resisting international military intervention in Syria.
The U.S. administration congratulated the Russian people for turning out to vote in big numbers in Sunday's election, but also expressed concern about allegations of fraud and urged a full investigation into the charges.
The State Department said Monday that the U.S. would work with Russia's "president-elect" once the votes are certified, but pointedly did not mention victor Vladimir Putin by name or offer any congratulations to Putin.
Communist Party candidate Gennady Zyuganov was a distant second in the election, followed by Mikhail Prokhorov, the billionaire owner of the New Jersey Nets basketball team whose candidacy was approved by the Kremlin in what was seen as an effort to channel some of the protest sentiment. Prokhorov attended Monday's protest.
The clownish nationalist Vladimir Zhirinovsky and socialist Sergei Mironov trailed behind. The leader of the liberal opposition Yabloko party was barred from the race.
Here's a list of the main violations reported during Sunday's ballot:
Voting outside designated polling stations
According to Golos, many voters cast their ballots either right at their state-run enterprises or under their bosses' watchful eyes, while others were bused to polling stations where they were not on voter rolls and where there was little oversight over voting.
Use of absentee ballots
Over 2 million absentee ballots were used in the election, and Golos monitors said that allowed for widespread violations. Many workers of state-controlled enterprises were forced to obtain absentee ballots and then vote under supervision. Absentee ballots were also used in multiple voting.
Voting more than once
Golos said "carousel voting," in which busloads of voters are driven around to cast ballots multiple times, also was widespread. Often, the offenders walked into polling stations wearing ribbons around their arms or with special marks in their passports, which they presented as identification. Election officials then identified those people as carousel voters and gave them the ballots of voters who were unlikely to show up.
A web-camera video from a village in the southern province of Dagestan shows several men standing at an electronic ballot box and feeding dozens of ballots into it for several minutes. Central Election Commission chief Vladimir Churov described the incident as the local election officials' "fatal mistake." Churov explained that those ballots had been filled out by sick or elderly people voting at home and that election officials were merely depositing them in the ballot box. In the end, voting results at that polling station were annulled after the complaints.
Paid to vote
Another video shows an activist who signed up on an Internet forum to take part in multiple (carousel) voting for 1,000 rubles ($33). The video, shot by the activist, shows him meeting the organizers at a subway station and then being bused to a nearby polling station. The activist and a dozen other people are shown lining up to receive the ballots and then voting at the station. Their names appear on a separate list at the polling station.
Not free or fair
Andrei Buzin, an election expert at Golos, said that the falsifications were not widespread enough to have left Putin with less than 50 percent of the vote and require a runoff, but the vote was still skewed. "I wouldn't call these elections free or fair," Buzin said.
The Organization of Security and Cooperation in Europe said "there was no real competition, and abuse of government resources ensured that the ultimate winner of the election was never in doubt." The watchdog noted that the election "process deteriorated during the vote count, which was assessed negatively in almost one-third of polling stations observed."