Ukraine's incumbent leader on Sunday warned the country's future was in danger as he failed to lure out a comedian tipped to become the next president into a debate before a high-stakes run-off vote.

The actor Volodymyr Zelensky's predictable no-show prompted President Petro Poroshenko to stage a one-man debate at Kiev's Olympic Stadium as several thousand supporters flooded a nearby square.

Inside, two podiums were set up at the stadium's make-shift press centre and a moderator set down rules in front of journalists.

Outside the sports arena that seats 70,000 the Ukrainian leader's supporters chanted "Poroshenko" and urged Zelensky to show up.

With one week to go before the vote, Poroshenko, 53, has been eager to spar with the 41-year-old political novice and accused his rival of avoiding hard-hitting questions.

He also said he would respect the "choice of the Ukrainian people" if Zelensky were elected president on April 21 but added he was worried for the country's future.

"I do not like that a presidential campaign in Ukraine looks like a silent movie," Poroshenko said, standing next to Zelensky's empty podium.

"I must say: Ukraine's fate is in danger," he said at the end of his hour-long appearance.

Poroshenko has been scrambling to recover lost ground in a bruising campaign as polls show Zelensky easily defeating the incumbent for the leadership of a country locked in a war with Moscow-backed separatists. 

Zelensky, whose political experience is limited to playing a president on TV, has shunned traditional rallies, instead performing satirical shows.

He however capitalised on frustration over mainstream politics, war with Kremlin-backed rebels, poverty and corruption to defeat Poroshenko in the first round of voting on March 31.

Support for the comedian among voters has doubled to 61 percent since then, with Poroshenko on 24 percent, according to the Rating pollster.

The Ukrainian president has touted himself as the only candidate able to stand up to Russian President Vladimir Putin and repeatedly urged the comic to hold several rounds of policy debates.

Zelensky has said he was ready to debate the Ukrainian president next Friday.

At the sports arena, Poroshenko repeated that debates were not scary.

"Do not be afraid," he taunted his rival.

"I have come, you are not here," Poroshenko added, citing a line from a humorous folk song.

Despite the flamboyant media spectacle that included alcohol and drug tests for the two candidates, the stakes are high for the country of 45 million seen as the biggest frontier between the European Union and Russia. 

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