* Putin almost certain extend his rule by six years
* Opposition plans protests after the election
* Opponents say the vote is not legitimate
* Putin facing growing fatigue after 12 years in power
VLADIVOSTOK, Russia, March 4 (Reuters) - Vladimir Putin is almost certain to win a third presidential term in an election that began on Sunday in Russia's far east, though opponents have challenged the legitimacy of a vote they say is skewed in his favour.
Putin's aides hope a strong win will take the sting out of an urban protest movement that casts the former KGB spy as an authoritarian leader who rules by allowing a corrupt elite to siphon off the wealth from the world's biggest energy producer.
In interviews from the Arctic to the shores of the Pacific Ocean, Russians gave a mixed picture: some expressed anger at being offered no real choice while others said Putin had proved he was a leader who could rule Russia.
"I shall of course vote for Putin. Who else is there?," said Mikhail, a student at the State University of Economics and Service in Vladivostok, a port city of 600,000 people on the Pacific and the biggest city in Russia's Far East.
"Only Putin is able to rule Russia," said Mikhail, who refused to give second name.
The last major opinion polls before the election showed Putin, who ruled as president from 2000 to 2008 and then as prime minister, was likely to win 59-66 percent of the vote, thus avoiding a runoff that would dent his authority.
Polls opened at 2000 GMT on Saturday in the icy tundra and sparsely-populated swathes of Russia's far east and close at 1700 GMT on Sunday in Russia's Western exclave of Kaliningrad, which is wedged between Poland and the Baltic Sea.
Victory is almost certain for Putin, a ruler lionised by state television and running against a cast of four politicians who, apart from tycoon Mikhail Prokhorov, have all made their careers by losing elections to the Kremlin.
But 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.
Russia's 59-year-old "alpha-dog" leader had to fight a tough campaign after initially misjudging the significance of the biggest protests of his 12 years in power.
The protests were sparked by a disputed Dec. 4 parliamentary election, but the anger was focused against Putin who bungled the Sept. 24 announcement of his presidential bid by appearing 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 on campaign as a statesman who can face off the chaos which 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.
Putin's campaign boosted his ratings by several percentage points, especially in the provinces, and drove up prices of Russian stocks and bonds as investors bet Putin will win the vote easily.
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 two decades after the 1991 fall of the Soviet Union.
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 skewed heavily in Putin's favour even without the vote rigging they expect.
Alexei Navalny, the most influential figure in the protest movement, has said Putin's election cannot be legitimate and called for an escalation of the protests including tent camps in central Moscow.
"If he does become president, he will not become a legal president, it will be an inherited throne," Navalny, a 35-year-old anti-corruption blogger told Reuters on his release from jail in December. He was detained during an protest after the parliamentary election and sentenced to 15 days in jail.
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 web site during the election.
But thousands of opposition activists as well as an international observer mission are also monitoring the polls. Exit polls will be released shortly after voting ends. (Writing by Guy Faulconbridge; Editing by Andrew Heavens)