Ukraine readied Saturday for a change in leadership as a young comedian appeared set to crush his incumbent rival in presidential polls, delivering a stinging rebuke to the old elite.

The 41-year-old television star Volodymyr Zelensky's bid to lead Ukraine began as a long shot but all polls show him defeating President Petro Poroshenko in a second-round vote on Sunday.

His victory would open a new chapter in the history of a country that has gone through two popular uprisings in two decades and is mired in a five-year conflict with Moscow-backed insurgents in the east.

"There is a chance for change. This man has potential," Vlad Motygulin, 33, told AFP in central Kiev on Saturday.

Others said they doubted the political novice's ability to enact real change as post-Soviet Ukraine's sixth president.

"I do not believe he will last long once he's elected," said Sergiy Fedorets, a 62-year-old pensioner.

"He has no support in parliament. He'll be eaten alive."

Poroshenko came to power shortly after a bloody 2014 uprising ousted a Kremlin-backed regime, triggering Moscow's annexation of Crimea.

But many in the country of 45 million people feel the promises of the uprising have not been fulfiled.

Zelensky has capitalised on anger over economic hardship and corruption, as well as his popularity as the star of the sitcom "Servant of the People", in which he plays a school teacher who becomes a president.

Analysts say his political programme is vague at best and it remains unclear who will fill key positions in his government.

Supporters, however, say only a new face can clean up the country's murky politics.

- Pompeo calls Zelensky -

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

US Secretary of State Mike Pompeo called both Zelensky and Poroshenko on Friday.

He "reiterated our commitment to working with whomever the Ukrainian people choose to ensure the success of a secure, prosperous, democratic, & free (country)," Washington's special envoy Kurt Volker said on Twitter.

Last week French President Emmanuel Macron hosted both candidates.

Poroshenko, 53, has fought hard to secure re-election but his pleas to forgive him for past mistakes and give him a second chance have fallen on deaf ears.

A survey by the Rating pollster released on Thursday showed Zelensky winning 73 percent of the vote against 27 percent for Poroshenko.

On Friday, the two rivals traded insults in the first and only policy debate of the campaign at Kiev's 70,000-seater Olympic Stadium in front of thousands of spectators.

Ukrainians said the head-to-head was more about political theatrics and a battle of wits than a much-needed policy debate.

- 'This is the end' -

Zelensky accused Poroshenko -- a tycoon who made a fortune in chocolate before taking office -- of enriching himself and failing to put an end to the war in the east that has claimed some 13,000 lives.

"I am the result of your mistakes and promises," he told the president, vowing to dismantle the old system.

Many in the crowd booed Poroshenko, shouting, "This is the end."

The Ukrainian leader attacked the inexperience of the untested Zelensky, calling him a draft dodger and doubting his ability to stand up to Russian strongman Vladimir Putin.

"An actor without any experience whatsoever cannot lead a war against an aggressor," an emotional Poroshenko said.

He also drew attention to Zelensky's close ties to controversial tycoon Igor Kolomoysky whose TV channel broadcasts the comedian's shows.

The vote frontrunner vowed to jail the tycoon if he violated any laws.

Supporters credit Poroshenko with rebuilding the army and securing an Orthodox Church independent of Russia but he won just over half of Zelensky's vote share in the first round of the election last month.