By Alissa de Carbonnel and Alexei Anishchuk
GROZNY, Russia, March 4 (Reuters) - Vladimir Putin sought a convincing victory in Russia's presidential election on Sunday to strengthen his hand in dealing with the biggest protests of his 12-year rule, but faced allegations of widespread fraud.
Opponents said the voting was heavily skewed to help the former KGB spy return to the Kremlin after four years as prime minister and vowed to step up three months of protests against him. Some voters said they were forced to vote for him.
But Putin's victory was not in doubt in voting from the Pacific coast to western borders with the European Union, and from the Arctic north to the frontier with China.
The man credited by many Russians with rebuilding the country's strong image and overseeing an economic boom in his 2000-08 presidency hoped to win outright in the first round and portray this as a strong mandate for six more years in power.
"I think the elections will be legitimate, fair, and Putin will win in the first round, unless the court rules otherwise," Putin's spokesman, Dmitry Peskov, was shown saying confidently on Internet and cable television channel TV Dozhd.
Early signs were that turnout would be high. Officials said more almost 48 percent of voters had cast their ballots by 3 p.m. Moscow time (1100 GMT), more than at this stage in the 2008 vote that elected Putin's ally, Dmitry Medvedev, to the Kremlin.
Some voters expressed anger at being offered no real choice in a vote pitting Putin against four weaker candidates - communist Gennady Zyuganov, nationalist Vladimir Zhirinovsky, former parliamentary speaker Sergei Mironov and billionaire metals tycoon Mikhail Prokhorov.
Others said Putin, 59, who has portrayed himself as a man of action and guardian of stability, was the tough national leader the world's biggest country and energy producer needed.
"I voted for Putin because he was a good president and our children were looked after and that's all. That's how I feel," said Maria Fedotova, a 92-year-old grandmother wrapped up in fur coat and hat, flanked by relatives.
Opinion polls showed Putin, who has remained Russia's dominant leader despite stepping aside in 2008 because he was barred from a third straight term by the law, would win 59 to 66 percent of the vote, enough to avoid a second-round runoff.
But some voters are tired of his macho antics and a system that concentrates power in his hands. They fear he could win two more terms, ruling until 2024 - almost as long as Soviet dictator Josef Stalin.
Others regard cheating as inevitable after allegations of fraud in a parliamentary poll win by Putin's United Russia party on Dec. 4.
"They are stealing our votes," said Valentin Gorshun, a patient in Moscow hospital number 19, where more than 90 percent of votes went to United Russia party in December.
"It is probably the same at all hospitals," he said. "I think they are preparing a huge falsification. Emperor Putin has decided everything."
Some state employees in the southern Chechnya region, where Putin's party won more than 99 percent suport in December, said they had been ordered by officials to vote for Putin.
"I came here ... because we were forced to come here and vote for Putin," said Zarf, who did not give her last name.
Elmira, a 27-year-old resident of Chechnya who also declined to give her name because she feared repercussions, said she had been told to round up people to vote for Putin.
"We were told to bring 10 people each to the voting. I have overachieved the plan," she said .
Putin, who voted in Moscow with his wife Lyudmila in a rare joint public appearance, dropped his ballot paper before voting and had to pick it up. Asked by reporters whether he ruled out a runoff, he said it would "depend on the voters."
Three women from a group specialising in naked protests were detained by police after stripping topless at the polling station, moments after Putin left, chanting: "Putin is a thief."
Vote monitors from the opposition and bloggers posted allegations of election rigging countrywide.
Television host Ksenia Sobchak said observers were barred from the St Petersburg district where she was helping out as a vote monitor. Golos, an independent monitoring group, said it had already registered 2,283 reports of violations nationwide.
An Interior Ministry spokesman said there had been no major violations. Election officials also dismissed reports of widespread fraud in a parliamentary election on Dec. 4 which triggered the opposition protests.
In an attempt to allay fears of vote rigging, Putin ordered 182,000 web cameras to be installed at 91,000 polling stations to stream footage of ballot boxes and vote-counting onto a website during the election.
A senior United Russia functionary said transparency of Russian elections should become an "example" for other countries to follow, Interfax news agency reported.
Thousands of opposition activists as well as an international observer mission were also monitoring the polls. Exit polls will be released shortly after voting ends.
Growing voter fatigue with Putin has unsettled Russia's elite of officials, former spies and billionaire businessmen. Putin's self-portrayal as the anchor of Russian stability hinges on his popularity.
He fought a tough campaign after initially misjudging the significance of the opposition protests.
They were sparked by the disputed Dec. 4 election, but the anger was focused at Putin, who bungled the Sept. 24 announcement of his presidential bid by appearing simply to inform Russians that he would rule for another six years.
Employing the rhetoric that helped transform President Boris Yeltsin's successor into one of the world's most powerful men, Putin cast himself during his campaign as a statesman who can face off the chaos that has laced centuries of Russian history.
He raced around some of Russia's 83 regions, berating minions in public for high prices, and mixed promises of increased budget spending with dark warnings of foreign plots.
But when he returns to the Kremlin, Putin will have to grapple with a mood change among many urban Russians who now view him as a hindrance to Russia's development.
Russia's opposition leaders, a fragmented group of activists, journalists and bloggers, are preparing rallies for the day after the vote and say the election is slanted in Putin's favour even without the vote rigging they expect.
Alexei Navalny, a prominent opposition leader and anti-corruption blogger, said Putin's election cannot be legitimate and called for more protests, including tent camps in Moscow.
"If he does become president, he will not become a legal president, it will be an inherited throne," Navalny said before the vote.