Zelensky, a 41-year-old actor best known in the country for playing the president in a television show, is favourite to become Ukraine's next leader after he dominated a first-round vote on March 31, despite many initially dismissing his candidacy as a joke.

Observers say Ukraine's two-round election is one of the country's most unpredictable votes since it gained independence from the Soviet Union in 1991.

It was Poroshenko who initially suggested a debate.

Ukraine's incumbent leader is eager to spar with his political novice rival, giving him a chance to show off his debating skills and outflank the comedian before a run-off vote on April 21.

But he had to agree to a number of unusual conditions set down by Zelensky, including a requirement for both of them to undergo medical tests to prove they do not abuse alcohol or drugs.

Zelensky also wanted the debate held at Kiev's Olympic Stadium, which seats over 70,000 people.

"(If you want) a stadium, let it be a stadium," Poroshenko said in a video address.

The 53-year-old also warned that an election debate was no laughing matter and a presidential election would determine the country's fate for decades to come.

"A debate is not a show," Poroshenko said in an apparent jab at Zelensky's lack of political experience.

"A debate is not staged for a stadium spectacle".

With 100 percent of the ballots counted, Zelensky is ahead with more than 30 percent of the first-round vote, nearly twice that of Poroshenko, on almost 16 percent.

Frontrunner Zelensky has shunned traditional rallies, instead performing satirical shows and blurring the line between reality and fantasy.

Despite limited political experience Zelensky capitalised on frustration over corruption and a stalling economy to leapfrog his establishment opponents.

Zelensky accepted a debate with Poroshenko after a viral campaign on social media urging him to publicly share his political opinions and proposals.

The comedian eventually agreed, adding the conditions, which also included the demand that Poroshenko should stop calling him a "clown" and a "puppet," a reference to the frontrunner's alleged political ties to oligarch Igor Kolomoisky.

Journalist and political observer Pavlo Kazarin said both men wanted to expose each other's weaknesses.

"The president's team wants to force Volodymyr Zelensky to start talking," he wrote. "Zelensky's staff wants to impose their own agenda on Poroshenko."

The increasingly sensational nature of the presidential campaign has divided Ukrainian society and set social media alight.

"Netflix is crying, HBO already realised that Game of Thrones should have had a different subject," Oksana Pavlenko, founder of an online magazine for women, wrote on Facebook, referring to the award-winning fantasy series.

"How about the Colosseum?" quipped investigative journalist Mykhailo Tkach, referring to Rome's ancient landmark.

But many said the debate -- if it ever takes place -- was a serious affair.

"It looks like the debates at the Olympic Stadium will become a political duel whose result will show whether the incumbent will be able to reverse the course of the campaign," tweeted Ukrainian lawmaker Mustafa Nayyem.

By law, televised election debates are to be held on the last Friday before the second-round runoff, which would be April 19.

The head of Zelensky's campaign Ivan Bakanov told Ukrainian media that it was premature to discuss the debate in detail, saying the date and time would be agreed later.

The last election debate between presidential candidates took place in 2004.