Ukrainian comedian Volodymyr Zelensky has won a landslide victory in the country's presidential election, exit polls suggest.

The polls give the political newcomer, who dominated the first round of voting three weeks ago, more than 70% support.

Mr Zelensky, 41, challenged incumbent president Petro Poroshenko who has admitted defeat.

The apparent result is being seen as a huge blow to Mr Poroshenko and a rejection of Ukraine's establishment.

"I will never let you down," Mr Zelensky told celebrating supporters on Sunday.

"I'm not yet officially the president," he added. "But as a citizen of Ukraine I can say to all countries in the post-Soviet Union: Look at us. Anything is possible!"

If polls are correct, he will be elected for a five-year term.

Ukraine's president holds significant powers over the security, defence and foreign policy of the country. (BBC)