KIEV, May 25 - 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 55 percent in exit polls on a first-round ballot marred by the reality that millions were unable to vote in the troubled eastern regions, Reuters reported.

Results will not be announced until Monday but runner-up Yulia Tymoshenko, with 13 percent, made clear she would concede, sparing the country a tense three weeks until a runoff round.

Poroshenko has no time to lose to make good on pledges to end "war" 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 post-Soviet mismanagement and chronic corruption.

The size of his victory reflects in part Ukrainians rallying behind the front-runner in the hope of ending a political vacuum that Russian President Vladimir Putin has exploited to annex the Crimea peninsula and offer solidarity, and maybe more, 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 Kiev's Independence Square, where pro-Western "EuroMaidan" protests ended in February in bloodshed that prompted President Viktor Yanukovych to flee to Russia. "I just want all of this to be over," she added. "I think that's what everybody wants."

In the eastern Donbas coalfield, where militants ensured polling stations were closed to some 10 percent of the national electorate, 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.

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 square the circle of turning 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, occupying a vast swathe of the borderlands between East and West.

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 Donbas - though quite when is unclear given that it may take some time to be formally sworn in - Poroshenko said he was ready to negotiate with anyone, and to offer the kind of 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 that he was ready to work with a new Ukrainian administration, Russia may use the gaps in the election in the east to question 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 recognize 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." (rt/ez)