Ukrainians are heading to the polls in a run-off election to pick the country's next president, informs bbc.com.
Voters face a stark choice between tycoon Petro Poroshenko, the incumbent president, and television comedian Volodymyr Zelensky, new to politics.
The TV celebrity is favourite in the polls, having dominated the first round of voting three weeks ago when 39 candidates were on the ticket.
Polling stations opened 08:00 (05:00 GMT) and will close 12 hours later.
A court in the capital, Kiev, has rejected a lawsuit calling for Mr Zelensky to be barred from standing.
A man had complained that the distribution of free tickets for a presidential debate by Volodymyr Zelensky's candidacy amounted to bribery.
Both candidates cast their votes in the capital Kiev.
Mr Poroshenko said the election was no less important than that of 2014, which followed the ousting of a pro-Russian administration.
"The name of the president to be elected is not important. Of special importance is the strategy and the path that our nation will continue to follow," he said.
After voting, Mr Zelensky said: "Today it will be the victory of Ukrainians, the victory of Ukraine, and - I hope - the victory of a fair choice."
On Friday the two candidates appeared at Kiev's Olympic stadium to debate for the first time.
The televised event was their first face-off after an usual campaign where Mr Zelensky has primarily used social media to communicate with the voting public
The winner of Sunday's vote will be elected for a five-year term as president.
The position holds significant powers over the security, defence and foreign policy of the country.