MOSCOW — Russian voters overwhelmingly granted Vladimir Putin a six-year term as president Sunday, setting the stage for a far more suspenseful postelection confrontation between Putin and opposition groups.

A small army of first-time election observers fanned out across Russian polling places, and hundreds of thousands of citizens watched ballot boxes via a vast $478 million network of Web cameras installed in polling places Sunday in what amounted to a huge experiment in public scrutiny of the voting process.

Evidence of brazen violations during parliamentary elections in December helped spark a series of unusual anti-government protests, and activists Sunday circulated evidence — much of it via video clips — that they said established a comparable level of foul play in the presidential election.

But this time, as the election approached, authorities broadcast television exposés about the observers, accusing them of fabricating reports of fraud in a foreign-backed effort to topple Putin's government.

Putin has been Russia's pre-eminent leader for 12 years, having served two terms as president from 2000 to 2008 before his current term as prime minister.

But the prospect of more protests, starting with a rally Monday night in Pushkin Square in central Moscow, threatened to undercut his promise of stability.

Some opposition leaders called for protests beyond those allowed by government permits, raising the prospect of a sharp response from the authorities.

The popular anti-corruption blogger, Aleksei Navalny, said he would lead an unsanctioned march to the Kremlin after Monday's rally. He has called for a permanent encampment of demonstrators like those created by some of the Occupy movements in the West.

"We know that the election results were falsified and we are not going to put up with Putin's usurping power like this!" Ilya Yashin, an opposition leader, said. "We will take to the streets and stay and we are ready for anything, even a crackdown."

Early returns showed Putin winning about 60 percent of the vote, comfortably above the 50 percent needed to avoid a runoff. Not long after the polls closed in Moscow, tens of thousands of Kremlin supporters gathered in Manexhnaya Square for a victory celebration and concert.

"We have won," Putin declared to the supporters right outside the Kremlin walls, a tear running down his cheek. "We have gained a clean victory!"

The voting took place under heightened vigilance. On Sunday, thousands of election observers took up posts across the country, most of them — in accordance with Russian election law — aligned with a candidate.

By some tallies, there were more than 3,000 complaints of violations, including "carousel voting," in which the same people cast ballots at multiple locations, and "centralized voting," in which managers of factories, schools, hospitals and other large organizations pressure employees to vote for a specific candidate. In some cases, ballots were collected at the workplace.

In one shot from the Web cameras at a polling place in Dagestan, in the Caucasus, two people are seen placing ballot after ballot in two separate voting machines. In another, recorded in Chechnya, a woman seems to vote twice. But activists argued that many of the Web cameras were either turned off or did not provide clear images.

And once again there were statistically improbable results from the North Caucasus, which is home to 6 percent of the Russian electorate and where Putin and his party have previously won close to 100 percent of the vote with abnormally high turnout.

Communist leader Gennady Zyuganov, also a candidate for president, declared the results illegitimate even before all the votes were counted. "It was illegitimate, unfair and not transparent," he said. "I will not congratulate anyone."

Putin faced three well-worn opponents he had defeated in the past — including Zyuganov — and one newcomer, Mikhail Prokhorov, a billionaire industrialist and owner of the New Jersey Nets basketball team, who mustered the 2 million signatures to get on the ballot but had no party to support him and no political experience.

Vladimir Zhirinovsky, 65, ran for the fifth time as the leader of a nationalist party; as well as Sergei Mironov, 59, of the Just Russia Party, who simultaneously ran against Putin and endorsed him in 2004.

Putin, who did little traditional campaigning and would not debate his opponents, nonetheless engaged in some of the most aggressive election-year politicking of his career. He postponed for six months the annual increase in household utility charges, the largest expense for most Russian families; increased pensions and military salaries; and promised an avalanche of new government spending.

He employed anti-American rhetoric, accusing the United States of trying to stoke revolution in Russia as well as in the Middle East. And he appealed to nationalist pride.

"We still have much to do for our Russia and for our people," he thundered at a rally. "And we will do it based on the talent of our people, on our great history, which is written with the blood and sweat of our forefathers."

On Sunday, as is usual on Election Day in this still relatively young democracy, tens of thousands of police officers were mobilized in a show of order and security.

Some monitoring efforts were simply shut down. A parallel count organized by opposition lawmaker Ilya Ponomaryov on Sunday had to be halted when the police evacuated the cafe where it was being held, saying there had been a bomb threat. The cafe was a last-minute location, because the Federal Guard Service had shut down the project's original location Saturday, Ponomaryov said.

Golos, Russia's most prominent election monitoring group, said the group had not documented the same widespread ballot-stuffing it had found in December, and speculated that was because "falsification technology has tilted in the direction of more complex and difficult to uncover."

Compiled from reports by New York Times correspondents David M. Herszenhorn, Ellen Barry and Sophia Kishkovsky. Information from Los Angeles Times is included.