* "Chocolate King" Poroshenko claims outright victory
* Exit poll score of 55 pct effectively rules out runoff
* Huge task to square pro-West move with Moscow
* Pro-Russia separatists kept millions from voting
* Separatists force closure of Donetsk airport in east (Updates preliminary results, adds Donestk airport closed)
By Richard Balmforth and Alastair Macdonald
KIEV, May 26 (Reuters) - Petro Poroshenko, a billionaire chocolate manufacturer, claimed the Ukrainian presidency with an emphatic election victory on Sunday, taking on a fraught mission to quell pro-Russian rebels and steer his fragile nation closer to the West.
A veteran survivor of Ukraine's feuding political class who threw his weight and money behind the revolt that brought down his Moscow-backed predecessor three months ago, the burly 48-year-old won more than 50 percent in preliminary results, against just 13 percent for his closest challenger.
The robust margin gives him a firm mandate, event though millions of Ukrainians were unable to vote in eastern regions prowled by armed pro-Moscow separatists.
Full results will not be announced until Monday, but runner-up Yulia Tymoshenko made clear she would concede, sparing the country a tense three-week wait for a runoff round.
Preliminary results with about 50 percent of votes counted gave Poroshenko 53.7 percent and former premier Tymoshenko 13.1.
Poroshenko, known as the "Chocolate King", has no time to lose to make good on pledges to end fighting with separatists in the Russian-speaking east, negotiate a stable new relationship with Moscow and rescue an economy sapped by months of chaos and 23 years of corrupt post-Soviet mismanagement.
The size of his victory reflects in part Ukrainians rallying behind the frontrunner in the hope of ending a political vacuum that Russian President Vladimir Putin exploited. Since March, Moscow has annexed Ukraine's Crimea peninsula and offered solidarity - and maybe more concrete support - to rebels in the east who want to break with Kiev and accept Russian rule.
"He has taken a heavy burden on his shoulders," said Larisa, a schoolteacher who was among crowds watching the results on Independence Square, where pro-Western "EuroMaidan" protests ended in bloodshed in February that prompted President Viktor Yanukovich to flee to Russia. "I just want all of this to be over. I think that's what everybody wants."
In the eastern Donbass coalfield, where militants shut polling stations cutting off some 10 percent of the national electorate from the vote, rebels scoffed at the "fascist junta" and announced a plan to cleanse their "people's republic" of "enemy troops".
A minister in Kiev said in turn its forces would renew their "anti-terrorist operation" after a truce during the polling.
More than 20 people were killed in the region last week.
On Monday, the international airport in Donetsk, the biggest city of the Donbass, closed after dozens of armed separatists visited the terminal in the early hours to demand the withdrawal of Ukrainian troops from the area. No violence was reported.
Claiming a popular mandate for a resumption of efforts to bind the nation of 45 million into association with the European Union - a drive that triggered the whole crisis six months ago - Poroshenko said he was ready to negotiate with Putin and called Russia a vital partner. He insisted Crimea must be returned.
Yet it remains unclear how the tycoon can turn firmly westward as long as Russia, Ukraine's major market and vital energy supplier, seems determined to maintain a hold over the second most populous ex-Soviet republic.
Nor is it clear that Poroshenko has new answers to resolving the uprising in the industrial east, given the weakness of his forces and the threat of Russian military intervention - a threat that has raised fears of a new Cold War, or worse, and has been met by only tentative U.S. and EU economic sanctions.
Declaring that his first trip would be to the Donbass - though quite when is unclear as it may take some time to be sworn in - Poroshenko said he was ready to negotiate with anyone, and to offer the regional autonomy, Russian language rights and budgetary powers that many want in the east.
"To people who have taken up arms but are not using them, we are ready to give amnesty," he told a news conference at which he fielded questions in a fluent mix of Ukrainian, Russian and English. "As for those who are killing, they are terrorists and no country in the world conducts negotiations with terrorists."
Poroshenko can be sure of a welcome from the European Union and United States - President Barack Obama hailed the election as a step toward restoring Ukrainian unity. But although Putin told an international audience at the weekend he was ready to work with a new Ukrainian administration, Russia could use the gaps in the election in the east to challenge its legitimacy.
A senior member of Putin's party, deputy parliamentary speaker Sergei Neverov, gave a taste of that when he wrote on Facebook: "It is hard to recognise the legitimacy of elections when tanks and artillery are wiping out civilians and a third of the population is driven to the polling stations at gunpoint."
He ridiculed Western leaders for endorsing the vote.
In a statement, Obama did not pre-empt the results by naming Poroshenko but praised Ukrainians for turning out to vote. He called the election "another important step forward in the efforts of the Ukrainian government to unify the country and reach out to all of its citizens to ensure their concerns are addressed and aspirations met".
Poroshenko served in a cabinet under Yanukovich and under previous governments led by Yanukovich's foes. This breadth of experience has given him a reputation as a pragmatist capable of bridging Ukraine's divide between supporters and foes of Moscow.
A former national security council chief, foreign minister and trade minister, he was a strong backer of the protests that toppled Yanukovich and is thus acceptable to many in the "Maidan" movement who have kept their tented camp in the capital to keep pressure on the new leaders to honour their promises.
Constitutional changes since Yanukovich's fall will leave Poroshenko with less power than his predecessor. He will share duties with Prime Minister Arseny Yatseniuk and parliament.
Poroshenko, who has worked closely with the liberal Yatseniuk in recent months, said there should be a parliamentary election before the end of the year - though he said it should not take place before conflict in the east was over.
Poroshenko and former prime minister Tymoshenko traded accusations of corruption when both were in government following the "Orange Revolution" of 2004-05 that thwarted Yanukovich's first bid for the presidency. But many voters saw him as less culpable than others of enriching himself illegally.
Where many "oligarchs" across the former Soviet Union took control of huge, formerly state-owned assets in the 1990s, many credit Poroshenko with building his Roshen confectionery empire himself. His other interests include a TV news channel.
"He's a serious person who built up his business himself," said Olga Netreba, 54, a civil servant in the industrial city of Dnipropetrovsk who said she had voted for him.
"I expect that with him Ukraine will be in Europe, that we will finally start getting towards Europe, so that living standards rise and Ukraine never returns to dictatorship."
Many people in the regions of Donetsk and Luhansk, home to close to 15 percent of the population, said they were frustrated at being unable to vote as polling station staff stayed home out of fear or found themselves attacked by pro-Russian militants.
Separatists claimed overwhelming support for a break with Kiev after makeshift referendums two weeks ago. But polls before the outbreak of violence that has cost dozens of lives suggested a majority wanted to remain in Ukraine, even if they complained of misrule from Kiev and feared anti-Russian bigotry among Ukrainian ultra-nationalists involved in the Maidan uprising.
"I don't think much of Kiev or the people's republic," said Ivan, 25, a security guard in the Donbass capital Donetsk. "Both sides say they want to bring order but it looks like neither can do it really. I don't think the election will change a thing.
"Someone first has to end this war - and I don't know who." (Additional reporting by Natalia Zinets, Pavel Polityuk, Gareth Jones and Anna Dabrowska in Kiev and Sabina Zawadzki, Gabriela Baczynska and Lina Kushch in Donetsk; Writing by Alastair Macdonald and Gareth Jones; Editing by Peter Graff)