Volodymyr Zelenskiy, a comedian with no political experience, was the top vote-getter in Ukraine's presidential election, an exit poll predicted.
It said he fell well short of the absolute majority needed to win outright in the first round.
The poll said incumbent President Petro Poroshenko was in a distant second place, closely followed by former Prime Minister Yulia Tymoshenko.
The election was shadowed by allegations of widespread vote buying. Police said they had received more than 1600 complaints of violations on voting day alone in addition to hundreds of earlier voting fraud claims, including bribery attempts and removing ballots from polling places.
Zelenskiy, who stars in a TV sitcom about a teacher who becomes president after a video of him denouncing corruption goes viral, led the field of 39 candidates with 30.4 per cent of the vote, according to an exit poll by the Kiev International Institute of Sociology and the Razumkov public opinion organisation.
Poroshenko tallied with 17.8 per cent support and Tymoshenko had 14.2 per cent, it said.
The top two candidates will face off in presidential runoff on April 21.
Final results are expected to be announced tonight NZT.
"Zelenskiy has shown us on the screen what a real president should be like," said voter Tatiana Zinchenko, 30, who cast her ballot for the comedian.
"He showed what the state leader should aspire for — fight corruption by deeds, not words, help the poor, control the oligarchs."
Campaign issues in the country of 42 million included Ukraine's endemic corruption, its struggling economy and a seemingly intractable conflict with Russia-backed separatists in eastern Ukraine that has killed 13,000 people since 2014.
Concern about the election's legitimacy have spiked in recent days after Ukraine's interior minister said his department was "showered" with hundreds of claims that supporters of Poroshenko and Tymoshenko had offered money in exchange for votes.
Like the popular character he plays, Zelenskiy, 41, made corruption a focus of his candidacy.
He proposed a lifetime ban on holding public office for anyone convicted of graft. He also called for direct negotiations with Russia on ending the conflict in eastern Ukraine.
"A new life, a normal life is starting," Zelenskiy said after casting his ballot in Kiev. "A life without corruption, without bribes."
His lack of political experience helped his popularity with voters amid broad disillusionment with the country's political elite.
"(We have) no trust in old politicians. They were at the helm and the situation in the country has only got worse — corruption runs amok and the war is continuing," said businessman Valery Ostrozhsky, 66, another Zelenskiy voter.
Poroshenko, 53, a confectionary tycoon when he was elected five years ago, pushed successfully for the Ukrainian Orthodox Church to be recognised as self-standing rather than a branch of the Russian church.
However, he saw approval of his governing sink amid Ukraine's economic woes and a sharp plunge in living standards.
Poroshenko campaigned on promises to defeat the rebels in the east and to wrest back control of Crimea, which Russia annexed in 2014 in a move that has drawn sanctions against Russia from the US and the European Union.
The president echoed his campaign promises of taking Ukraine into the EU and Nato. He said holding a fair, free election was "a necessary condition for our movement forward, to Ukraine's return to the European family of nations," and was confident about the ballot despite the bribery allegations.
The President's priorities persuaded schoolteacher Andriy Hristenko, 46, to vote for him.
"Poroshenko has done a lot. He created our own church, bravely fought with Moscow and is trying to open the way to the EU and Nato," Hristenko said.
Ukraine's former prime minister, Tymoshenko, shaped her message around the economic distress of millions in the country.
"Ukraine has sunk into poverty and corruption during the last five years, but every Ukrainian can put an end to it now," she said.
During the campaign, Tymoshenko denounced price hikes introduced by Poroshenko as "economic genocide" and promised to reduce prices for household gas by 50 per cent within a month of taking office.