Ukraine's rival presidential candidates have held a long-awaited head-to-head televised debate at Kiev's giant Olympic stadium.

Comedian Volodymyr Zelensky and incumbent president Petro Poroshenko shook hands before launching scathing attacks on one another.

Mr Zelensky is the clear favourite for Sunday's second-round vote, polls say.

But Mr Poroshenko argues that he is a political novice unfit for such a demanding job.

Mr Zelensky has relied on a slick social media campaign to tap into the frustration of many younger Ukrainians with establishment politicians, cronyism and corruption.

The debate in the capital's 70,000-seat stadium - where Mr Poroshenko's supporters appeared to outnumber Mr Zelensky's - was the first and only direct clash between the two rivals in the campaign.

The stadium had been divided into two halves for the debate, with a stage for each candidate to speak from at each end of the pitch - but Mr Poroshenko and Mr Zelensky ended up speaking on the same stage.

What did they say?

"I am not a politician," said Mr Zelensky, who spoke first. "I am just a simple person who has come to break down this system."

Mr Poroshenko responded by accusing his rival of avoiding active-duty military service and said he would not be able to stand up to Russian President Vladimir Putin.

"We only have a beautiful and bright sweet wrapper in which everyone can find what he is looking for," he said.