A comedian with no political experience won a landslide victory in Ukraine's presidential election on Sunday, exit polls showed, dealing a stunning rebuke to the country's political establishment.

Volodymyr Zelensky, whose only previous political role was playing the president on television, trounced incumbent Petro Poroshenko by taking 73 per cent of the vote, according to exit polls conducted by several think tanks.

Poroshenko lost to the television star across all regions of the country, including in the west where he traditionally enjoyed strong support.

It was an extraordinary outcome to a campaign that started as a joke but struck a chord with voters frustrated by poverty, corruption and a five-year war that has claimed some 13,000 lives.

The 41-year-old star of TV series “Servant of the People” will now take the helm of a country of 45 million people beset by challenges and having run on the vaguest of political platforms.

After taking the most votes in last month's first-round election, Zelensky had enjoyed a strong lead over the 53-year-old Poroshenko going into Sunday's poll.

Voting earlier in the capital Kiev, the beaming frontrunner had said his campaign managed to bring Ukrainians together.

“We have united Ukraine,” he said, wearing a casual suit with a t-shirt and accompanied by his wife.

“Everything will be all right.”

Sparkling wine was on offer at his campaign headquarters as the star's team prepared to toast his victory on Palm Sunday, a week before Orthodox Easter.

Preliminary results were expected in several hours but the same exit polls were accurate in the first round.

From Ukrainian-speaking regions in the west of the country to Russian-speaking territories in the war-torn east, many voters said they feared uncertainty but yearned for change.

“We're tired of all the lies,” said Marta Semenyuk, 26, who cast her ballot for the comedian.

“I think it just cannot get any worse and I hope he'll live up to his promises,” said Larisa, an 18-year-old student from thegovernment-held eastern port city of Mariupol.

Zelensky's victory opens a new chapter in the history of a country that has gone through two popular uprisings in the last 20 years and is mired in a conflict with Moscow-backed separatists in the east.

His supporters say only a fresh face can clean up Ukraine's politics and end the separatist conflict.

But others doubt the showman will be able to take on the country's influential oligarchs, negotiate with the likes of German Chancellor Angela Merkel and stand up to Russia's Vladimir Putin.

“People have gone mad, “Viktoriya Olomutska, a 39-year-old Poroshenko supporter, said in Kiev. “Cinema and reality are two different things.”

'Not funny'

Poroshenko mocked his rival's lack of political experience and argued he was unfit to be a wartime commander-in-chief.

The outgoing leader came to power after a 2014 pro-Western uprising ousted a Kremlin-backed regime, triggering Moscow's annexation of Crimea.

His supporters credited him with rebuilding the army and securing an Orthodox Church independent of Russia.

But many feel like the country's ruling elite have forgotten the promises of the revolution.

Earlier Sunday, Poroshenko warned Ukrainians against taking a chance on Zelensky.

“Because this is not funny. Well, at first it can be a bit funny and then it might hurt afterwards,” he said after casting his ballot.

The comic shunned traditional campaign rallies and instead performed comedy gigs and used social media to appeal to voters.

The Ukrainian president has strong powers over defence, security and foreign policy but needs backing from parliament to push through reforms.

Poroshenko's faction has the most seats in the current legislature and new parliamentary polls are due in October.

The West has closely watched the race amid concern a new government might undo years of economic reforms.

US Secretary of State Mike Pompeo called both Zelensky and Poroshenko on the eve of the run-off vote.