* Ukrainians vote in first-round presidential poll on Sunday
* Of 36 million electors, revolt may block 5 million in east
* Putin says to "respect" new leader likely to favour West
* Chocolate magnate Petro Poroshenko, 48, could win outright
* Voters want break with graft, poverty, but few new faces (Adds voter quotes, Tymoshenko quote, other details)
By Pavel Polityuk and Sabina Zawadzki
KIEV/DONETSK, Ukraine May 25 (Reuters) - Ukrainians voted on Sunday in a presidential election billed as the most important since they won their independence from Moscow 23 years ago, but armed pro-Russian separatists disrupted voting in eastern regions of the former Soviet republic.
Early signs pointed to a high turnout in sunny weather in an election where the main candidates, including front-runner Petro Poroshenko, a confectionery magnate, are promising closer ties with the West in defiance of Russia's President Vladimir Putin.
But the absence of over 15 percent of the electorate, in Russian-annexed Crimea and two eastern regions where fighting with pro-Moscow rebels continued on Saturday, may mar any result - and leave the Kremlin questioning the victor's legitimacy, for all Putin's new pledge to respect the people's will.
Voting began in most of Ukraine at 8 a.m. (0500 GMT) and will end 12 hours later, when exit polls will indicate a result ahead of an official outcome on Monday.
Only about 20 percent of the polling stations in the heavily industrialised, Russian-speaking Donetsk region, which has 3.3 million registered voters, were working as of 9:30 a.m. (0630 GMT), authorities said. None were open in the city of Donetsk.
"These are extremely important elections. We have to make sure Ukraine becomes a truly independent country, a powerful independent state that nobody will be able to push around," said pensioner Mikhailo Belyk, 65, casting his ballot at a crowded polling station in a southeast district of the capital Kiev.
Sounding an equally upbeat note, businessman Viktor Sypchenko, 45, said: "I am voting for my children and their future. I hope we can break free from our awful past."
The picture emerging in the east was more confused. European election monitors have largely pulled out of the Donetsk region for their own safety, citing a campaign of "terror" by pro-Russian separatists against Ukrainian electoral officials.
'THINGS ARE BAD'
At a school in a Donetsk suburb, pensioner Grigory Nikitayich, 72, was unhappy about being denied the right to vote for Poroshenko. "I don't even know where I can vote. No one has said anything. What kind of polls are these? Things are bad."
Others also complained of being prevented from voting, in some cases because ballot papers had not been delivered due to security concerns after at least 20 people were killed in the region during fighting over recent days.
Polls make Poroshenko, known as the "chocolate king" because of his confectionery empire, overwhelming favourite to win Sunday's election. The biggest question is whether he can take over 50 percent to win outright. If not, a run-off vote will be held on June 15.
He was a strong backer of the protests against Moscow-backed president Viktor Yanukovich last winter and has sought a quick victory by warning that new unrest might prevent a second round.
His closest, if distant, rival is Yulia Tymoshenko, a former prime minister. She remains a divisive figure to many, more closely linked than Poroshenko with the economic failures and graft that have blighted post-Soviet Ukraine.
"It is time to hold a referendum on joining NATO to restore peace in Ukraine," said Tymoshenko after voting in her native city of Dnipropetrovsk in central Ukraine. Russia is fiercely opposed to Ukraine joining the Western military alliance.
As Yanukovich's fiercest rival, Tymoshenko may benefit from the fact that few of the 5 million voters in his eastern power base regions of Donetsk and Luhansk may be able to cast ballots for any of the 21 candidates.
Interim Prime Minister Arseny Yatseniuk urged Ukrainians to hand the new president a strong mandate to forge closer ties with the European Union and move Ukraine away from a "grey zone of lawlessness and dark forces that dream of suffocating us and into ... a place where it is easier to breathe".
The West has backed the interim government since mass street protests toppled Moscow-backed Yanukovich in February.
But Russia characterised the protests as a "fascist coup". Citing the need to defend Ukraine's large Russian-speaking population, it seized Crimea and backed the separatists in eastern Ukraine.
The West hopes that a new leader in Kiev can help resolve a confrontation with Russia that has sparked military buildups east and west of Ukraine and raised fears of a new Cold War.
Putin pledged on Saturday to "respect" the people's choice and work with Ukraine's new administration - a conciliatory move during an economic forum at which he had acknowledged that U.S. and EU sanctions over Ukraine were hurting the Russian economy.
But he defended his annexation of Crimea in March as a response to the democratic will of the majority ethnic Russian population there. Kiev and its Western allies accuse Moscow of a propaganda war to sow fear among Russian-speakers in eastern and southern Ukraine of "fascist" Ukrainian nationalists and of supporting rebel forces who have seized many towns in the east.
Two weeks ago, separatists in the Donetsk and Luhansk regions ran referendums they said let them break from Kiev and opened a way to possibly following Crimea into union with Russia - though Moscow denies any plan to seize any more Ukrainian territory.
Opinion polls before the last few months of violence showed disillusion with Kiev's politicians in the east but limited appetite for outright secession.
Putin played down talk of a return to Cold War with the West and dismissed the idea he was bent on restoring the former USSR, whose collapse he has in the past lamented.
Washington and its EU allies are concerned that while Russia may accept the election result, it may use influence in eastern Ukraine to undermine the new president's authority and keep the country beholden to Moscow. Russian officials have questioned the value of holding the vote when the east is in "civil war".
A territory on a par with France and with 45 million people, Ukraine is the second most populous ex-Soviet state and plays a pivotal role in relations between Russia and the EU.
Large volumes of Russian natural gas flow across it to Germany and other consumers, creating mutual dependencies that complicate diplomatic calculations on all sides of the conflict.
The inheritor of a patchwork of regions ruled not only from Moscow but by Poland, Austria and others, Ukraine's mix of Russian and Ukrainian speakers as well as ethnic minorities have struggled to forge a common national purpose. But polls consistently show a majority in favour of independence.
Since the 2004 "Orange Revolution" that kept Yanukovich from power, Ukrainians of all stripes have been disappointed with a decade of economic drift and graft that won them the dubious distinction of being named Europe's most corrupt country. Their hopes for a fresh start are pinned on Sunday's vote.
Few of the leading candidate are new faces, however.
Both Poroshenko and Tymoshenko played leading roles in the administrations that preceded Yanukovich's defeat of Tymoshenko in the 2010 election. Poroshenko, now a burly 48-year-old, later held a cabinet post for a time under Yanukovich.
Both became wealthy in the anarchic post-Soviet 1990s, Poroshenko, now worth $1.3 billion according to Forbes, through his candy and chocolate empire, Tymoshenko as the "gas princess" involved in the trade and transit of Russian natural gas.
After the Orange Revolution, when he was head of the National Security Council and she prime minister, the two traded accusations of corruption. Tymoshenko, 53, was jailed in 2011 for corrupt gas deals with Russia but was released when Yanukovich was toppled and her record cleared. (Additional reporting by Gabriela Baczynska in Mariupol, Gareth Jones, Alastair Macdonald and Yvonne Bell in Kiev; Writing by Gareth Jones and Alastair Macdonald Editing by Ron Popeski and Jason Neely)