MOSCOW—Vladimir Putin claimed victory in Sunday's presidential election, but allegations of widespread fraud highlighted the challenge he faces as he seeks to extend his control over Russia for another six years.
Exit polls showed Mr. Putin was likely to win 58% to 59% of the vote against a field carefully screened to eliminate serious rivalry in the wake of the biggest anti-Kremlin protests in two decades. With more than 60% of the vote counted, official results showed Mr. Putin with 64.6%
"We have won in an open and fair fight," Mr. Putin declared at a celebration in Manezh Square near the Kremlin. "We have shown that our people can easily tell apart the desire for novelty and renewal from political provocations that have only one goal in mind—to break up the Russian state and to usurp power.
"I promised you we would win," he told tens of thousands of supporters, who chanted "Putin! Putin!" during his brief speech. "Glory to Russia."
Mr. Putin's words were unlikely to dispel questions over his legitimacy and whether he has the broad support needed to prolong a regime he has led since 1999 as president or prime minister.
"It's not an election," said Alexei Navalnyi, the anticorruption blogger and activist who has led the recent mass protests. "Putin had a chance to make at least the counting fair, but he didn't. Tomorrow we'll wake up in a country where a large chunk of the society doesn't see Mr. Putin as a legitimate president."
Allegations of fraud in the Dec. 4 parliamentary elections triggered the biggest antigovernment demonstrations since the collapse of the Soviet Union. Tens of thousands of protesters, many from the previously apolitical middle class that has grown during the relative prosperity of Mr. Putin's years in power, have taken to the streets to demand fair elections and call on Mr. Putin to step down.
Protest leaders said Sunday's election was unfair not only because of vote rigging but also because bureaucratic hurdles kept genuine opposition parties off the ballot. They had encouraged their followers to vote for any of Mr. Putin's four Kremlin-approved rivals.
The loosely organized protest movement now faces a test of its ability to keep up the pressure. "We have to find means to win over more people" to back demands for new parliamentary and presidential elections, Mr. Navalnyi said. "It will be peaceful but resolute action," he said, starting with a Monday evening rally in downtown Moscow.
Authorities denied any serious irregularities in Sunday's vote and brought thousands of extra police into the capital in preparation for more protests. On Sunday police occupied the city center as an estimated 35,000 people, many bused from provincial towns, turned out for Mr. Putin's victory party.
They were funneled into a tight group near the Kremlin walls. State-run television cameras panned overhead, showing panoply of flags that accentuated the size of the crowd.
Mr. Putin, hatless in a frigid wind that caused his eyes to tear up, appeared on a stage at 11 p.m., three hours after the polls had closed and with about a third of the vote counted. He was accompanied by President Dmitry Medvedev, the protégé who took over the presidency in 2008 after term limits obliged Mr. Putin to step aside.
Some in the crowd said they had come in support of Mr. Putin. Others said their employers had ordered them to come. Officials denied any coercion. Many in the crowd refused to give their names.
Danil Ignatov, a demonstrator in his 20s, called Mr. Putin "the only candidate worthy of this post. He has restored the economy. Because of him, Russia has risen from its knees."
Mr. Putin's expected vote total fell far short of the 71% he received in 2004 in winning a second presidential term. He will return to the office in a weaker position than he left the office in 2008 and became prime minister.
The December protests caused a dip in his approval ratings. But he recovered with an aggressive campaign, portraying himself as a man of action, a guardian of stability and a tough leader who could stand up to the West, whom he accused of abetting the protests in a bid to undermine Russia.
Mr. Medvedev responded to the protests by offering a political overhaul, including a return to elections for regional governors and easier registration requirements for opposition parties. On Sunday Mr. Putin's spokesman, Dmitry Peskov, endorsed those changes but emphasized that they would come gradually. "The political system needs to catch up a little bit to the level of development of our civil society," he told a Russian radio station.
Tensions with the U.S.—sparring over Syria, missile defense in Europe, and other issues—are expected to simmer under Mr. Putin. On Sunday, an Internet user mimicking U.S. Ambassador Michael McFaul's Twitter account tweeted that evidence of mass violations would undermine the election's legitimacy.
The message drew indignant responses from Russian officials before Mr. McFaul dismissed it as a fake.
Mr. Putin voted near his Moscow home in a rare appearance with his wife. Moments after he had left the polling station, three Ukrainian women from FEMEN, a group specializing in nude protests, entered, stripped to the waist and tried to steal the ballot box. They were arrested. A court sentenced them to jail terms ranging from 5 to 12 days for hooliganism.
Mr. Putin's backers said the vote showed that multitudes of recent protesters represent a small fringe of wealthy urbanites disconnected from the rest of Russian society.
Vyacheslav Nikonov, a political analyst who supports Mr. Putin said the opposition campaign even helped Mr. Putin by shoring up what he called the Russian leader's "soft, unstable electorate."
Leaving a Moscow polling station, Timur Shakarishvili, a 56-year-old bank worker, and his wife, Lyubov, said they voted for Mr. Putin and voiced disdain for the young protesters. "They're not like the people of our generation, who worked all their lives and never had time to have protests," said Ms. Shakarishvili, a 60-year-old retired industrial technician.
But opinion polls show a growing disenchantment with the Russian leader and the prior decade's implicit social contract, during which time the Kremlin steadily rolled back democratic freedoms but Russians enjoyed unprecedented prosperity.
"There's a steady fading of the hopes or illusion that this kind of super-centralized, authoritarian system can deliver steady economic development over the long term," said Lev Gudkov, director of the Levada Center, an independent pollster.
Gennady Zyuganov, the Communist leader, was in second place Sunday with 18% of the vote, according to the exit polls, which were conducted by the Vtsiom and Public Opinion Foundation pollsters. He was followed by billionaire tycoon Mikhail Prokhorov at 9%; he ran on a pro-business platform but struggled to shake allegations he was a stalking horse for the Kremlin. Ultranationalist Vladimir Zhirinovsky came in third with 7 to 8%, followed by socialist Sergei Mironov with 4 to 5%.
Tens of thousands of Russians signed up as volunteer poll observers, and there were numerous reports of violations Sunday.
Mr. Navalny said independent monitors who observed the vote count reported Mr. Putin falling short of a majority in Moscow and other large cities. Golos, an independent poll-monitoring group that has been accused by the Kremlin of bias against the government, posted claims of about 1,500 electoral violations on its Web site.
Yabloko, an opposition party whose candidate wasn't allowed onto the ballot, said its observers had found a busload of voters that cast ballots at six different Moscow precincts, in what is known in Russian electoral practice as a "carousel." Golos reported similar incidents in Novosibirsk, Russia's third largest city. Yabloko also reported precinct officials using erasable ink on official vote-tally documents.
The Interior Ministry said it hadn't found any violations that would cast doubt on the outcome of the vote.