Vladimir Putin sought a convincing victory in Russia's presidential election on Sunday to strengthen his hand in dealing with the biggest opposition protests since he rose to power 12 years ago.

Listing alleged irregularities online, opponents said the voting was marred by fraud and skewed to help the former KGB spy return to the Kremlin after four years as prime minister, and vowed to step up protests after the election.

But Putin's victory was not in doubt in voting across the icy tundra, large swathes of unpopulated territory and heavily industrialised urban centres from the Pacific coast to the western borders with the European Union.

The man credited by many Russians with rebuilding the country's strong image and overseeing an economic boom in his first presidency hoped to win outright in the first round and portray this as a strong mandate for six more years in power.

Early signs were that turnout would be high. Officials said more than 30 percent of voters had cast their ballots by 1 p.m. Moscow time (0800 GMT), more than in the 2008 election that brought 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 will vote for Putin... the country needs a strong leader, army serviceman Dmitry Samsonov, 23, said on a cold day in Moscow. They didn't let any other candidates through... There is no one else to vote for but Putin.

Opinion polls showed Putin, who was president from 2000 to 2008 before constitutional limits barred a third straight term, 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.

Pyotr Kirillov, 75, said in the Urals city of Yekaterinburg that he had not changed his ideals in the past 50 years despite the collapse of the Soviet Union, and voted for the communists.

Margarita Alyoshina, a 62-year-old Muscovite, said she would not vote for Putin so that he did not become complacent.

I won't vote for Putin just to make him think a bit, show that he is not the only man in the world. But anyway he will win, she said.

NAKED PROTEST

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 in the St Petersburg district where she was helping out as a vote monitor. Golos, an independent monitoring group, said it already registered 2,030 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 December 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.

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.

PUTIN FATIGUE

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 December 4 election, but the anger was focused at Putin, who bungled the September 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.

Putin 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.

Navalny has 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.

(Writing by Guy Faulconbridge and Alexei Anishchuk; Additional reporting by Nastassia Astrasheuskaya and Alissa de Carbonnel in Moscow; Editing by Timothy Heritage)