With nearly all ballots counted in the run-off vote, Mr Zelensky had taken more than 73% with incumbent Petro Poroshenko trailing far behind on 24%.

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

Russia says it wants him to show "sound judgement", "honesty" and "pragmatism" so that relations can improve. Russia backs separatists in eastern Ukraine.

The comments came from Russian Prime Minister Dmitry Medvedev, in a Facebook post on Monday.

He said he expected Mr Zelensky to "repeat familiar ideological formulas" that he used in the election campaign, adding: "I have no illusions on that score.

"At the same time, there is a chance to improve relations with our country."

Mr Poroshenko, who admitted defeat after the first exit polls were published, has said he will not be leaving politics.

He told voters that Mr Zelensky, 41, was too inexperienced to stand up to Russia effectively.

Mr Zelensky, a political novice, is best known for starring in a satirical television series Servant of the People, in which his character accidentally becomes Ukrainian president.

Zelenskiy, who plays a fictitious president in a popular TV series, is now poised to take over the leadership of a country on the frontline of the West’s standoff with Russia following Moscow’s annexation of Crimea and support for a pro-Russian insurgency in eastern Ukraine.

Declaring victory at his campaign headquarters to emotional supporters on Sunday night, Zelenskiy promised he would not let the Ukrainian people down.

“I’m not yet officially the president, but as a citizen of Ukraine, I can say to all countries in the post-Soviet Union look at us. Anything is possible!”

Zelenskiy, whose victory fits a pattern of anti-establishment figures unseating incumbents in Europe and further afield, has promised to end the war in the eastern Donbass region and to root out corruption amid widespread dismay over rising prices and sliding living standards.

But he has been coy about exactly how he plans to achieve all that and investors want reassurances that he will accelerate reforms needed to attract foreign investment and keep the country in an International Monetary Fund program.

“Since there is complete uncertainty about the economic policy of the person who will become president, we simply don’t know what is going to happen and that worries the financial community,” said Serhiy Fursa, an investment banker at Dragon Capital in Kiev.