Ukrainian confectionery tycoon Petro Poroshenko has claimed outright victory in the country's presidential election.
Mr Poroshenko, known as the "chocolate king", won more than 55% of the vote in the first round, exit polls suggest.
Announcing he had won, the 48-year-old businessman promised to forge closer links with the EU and restore peace in restive eastern regions.
Pro-Russian separatists severely disrupted voting there. Some 20 people have died in fighting in recent days.
No polling stations were open in Donetsk city, and across the region only seven out of 12 district electoral commissions were operating. The separatists are in control of large areas of the Donestk and Luhansk regions.
Four hours before polls closed, at 16:00 (13:00 GMT), unofficial estimates put the turnout nationwide at 45%.
Analysis by David Stern, BBC News, Kiev
The hope is that this election will unite the country and give Mr Poroshenko legitimacy - especially in the eyes of the people in the east, where the pro-Russia separatist movement has taken hold.
Just a handful of election commissions were operating in eastern regions, and there are fears that the coming days will see an escalation in violence there. The main question is - what will Russia's reaction be?
President Vladimir Putin has said Moscow will respect the election results. But respect is not quite the same as recognition. Russia may decide that any actions by Mr Poroshenko to put down the insurgency in the east will be unacceptable.
We've also heard that Mr Poroshenko could be a man Moscow can work with. He's a businessman with interests in Russia. The Russians have some familiarity with him, which can possibly contribute to negotiations in the coming days.
Addressing supporters in Kiev, Mr Poroshenko said he would support a parliamentary election later this year.
He also said he would never recognise Russia's "occupation of Crimea", annexed by Moscow in March. Asked about relations with Russia, he said the "sovereignty and territorial integrity" of Ukraine mattered most to him.
Mr Poroshenko is the billionaire owner of the Roshen chocolates group, a TV station and several manufacturing plants.
US President Barack Obama hailed the election as an "important step forward in the efforts of the Ukrainian government to unify the country".
Sunday's poll was called after President Viktor Yanukovych was deposed in February, amid mass protests against his pro-Russian policies.
Local elections were also held on Sunday in Ukraine. Former boxer Vitaliy Klitschko - again quoting exit polls - claimed he had been elected mayor of Kiev.
Mr Klitschko, a pro-Western politician, withdrew his own presidential bid and announced his support for Mr Poroshenko.
On Friday Russian President Vladimir Putin said he would respect the outcome of Ukraine's election.
Kiev and the West accuse Russia of stoking separatist sentiment - a claim President Putin denies.
Separatists in Donetsk and Luhansk declared independence after referendums on 11 May, a move not recognised by Kiev or its Western allies.
The two regions took their cue from a disputed referendum in Crimea, which led to Russia's annexation of the southern peninsula.
Eighteen candidates were competing in the presidential election, seen as a crucial to unite the country.
The exit polls suggest ex-PM Yulia Tymoshenko came a distant second, with over 12% of the vote.
"I want to congratulate the whole of Ukraine since despite external aggression, despite the Kremlin's intent to disrupt this election we had an honest and democratic election in Ukraine," Ms Tymoshenko said after the polls closed.
If the exit polls are confirmed, there will be no need for a run-off vote next month.
In other developments, Italian photojournalist Andrea Rocchelli and his Russian colleague Andrey Mironov were killed in clashes between separatist and government forces near Sloviansk on Saturday.
Meanwhile, Russian PM Dmitry Medvedev has begun a two-day trip to Crimea in a visit denounced by Ukraine as a "provocation".