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. Official results are expected to come in throughout Sunday night.

Mr Zelensky is best known for starring in a satirical television series in which his character accidentally becomes Ukrainian president.

The president holds significant powers over the security, defence and foreign policy of the country.

Analysis by Jonah Fisher, BBC News, Kiev

Ukraine's choice was between an experienced politician with five years as president on his CV and a comedian wielding little more than a blank sheet of paper. That so many people have opted for Volodymyr Zelensky is a humiliation for Petro Poroshenko.

Thirty-seven candidates were removed from the ballot paper from the first round and yet the president only picked up about 9% more votes this time. Mr Zelensky gained almost 45%.

This feels like a massive protest vote and for now Mr Zelensky and his campaign team are celebrating.

It's hard to see the feeling lasting long. The hard work will come when they have to start fleshing out what are at the moment vague policies.

It's one thing to have bold ideas but quite another to implement them

Polls gave Mr Poroshenko, who has been in power since 2014, 25% of the vote.

"The outcome of the election leaves us with uncertainty [and] unpredictability," he said after exit polls were released.

He added: "I will leave office but I want to firmly stress - I will not quit politics."