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:
Dear Vlad, Surprise! Surprise! You won. The #Russian people are crying too!— John McCain (@SenJohnMcCain) March 5, 2012