Ukrainians are going to the polls to vote in a new president after months of unrest following the ousting of former President Viktor Yanukovych.
There are 18 candidates on the ballot, which is widely seen as a crucial moment to unite the country.
But pro-Russian separatists have disrupted voting in the east, stopping many polling stations from opening and smashing up ballot boxes.
Some 20 people have been killed in fighting in recent days.
Voting in the eastern regions of Donetsk and Luhansk has been seriously disrupted.
There were no polling stations open in Donetsk city, and across the region only seven out of 12 district electoral commissions were operating.
The presidential elections were called after the last elected President, Viktor Yanukovych, was deposed in February amid mass protests against his pro-Russian policies.
Confectionary tycoon Petro Poroshenko, known as the "chocolate king", is the favourite to win.
Former Prime Minister Yulia Tymoshenko is lagging behind Mr Poroshenko in opinion polls.
If no candidate wins more than 50% of the vote, a second round will take place in June.
Prime Minister Arseniy Yatsenyuk urged people to vote and "defend Ukraine".
In an unprecedented move, Russian President Vladimir Putin said on Friday that he would respect the outcome of the election and was prepared to work with whoever was elected president.
Kiev and the West accuse Russia of stoking separatist sentiment – a claim President Putin denies.
The Organization for Security and Co-operation in Europe deployed 1,000 observers but withdrew its teams from Donetsk over fears for their security.
Separatists in Donetsk and Luhansk declared independence after referendums on 11 May, a move not recognised by Kiev nor its Western allies.
The two regions took their cue after a disputed referendum in Crimea, which was annexed by Russia in March.
Polling stations will remain open until 20:00 (17:00 GMT), with definitive results expected on Monday.