Conceding defeat Sunday evening, Poroshenko offered his congratulations to Zelenskiy, and added, "I will leave the office, but I want to announce firmly: I will not leave politics."

Poroshenko later wrote on Twitter: "You may just look at the celebrations in the Kremlin on the occasion of the elections. They believe that with a new, inexperienced Ukrainian President, Ukraine could be quickly returned to Russia's orbit of influence."

Zelenskiy combined a slick social media campaign with his image as an outsider, capitalizing on a swell of public anger at the old Ukrainian politics of cronyism and corruption.

Ultimately, the slow pace of reforms cost Poroshenko a second term, said political analyst Anatoliy Oktysyuk of the Kyiv-based Democracy House.

"Ongoing corruption scandals, some reforms, the decline in living standards and poverty prompted voters in east, in the center, and the west -- but also some in Kyiv -- to express their mistrust of politicians. Zelenskiy's victory is a referendum on the mistrust of former politicians," Oktysyuk added.

Critics highlight Zelenskiy's ties to exiled oligarch Ihor Kolomoisky, who is accused of bank fraud and who owns the 1+1' TV channel that broadcasts his show.

Zelenskiy has pledged to continue Ukraine's pro-Western path, five years on from Russia's forceful seizure of Crimea and invasion of eastern Ukraine.

"I think that a more pragmatic model of cooperation with Russia will be installed, that an economic cooperation will gradually be reinstated. But the conflict will not be resolved. Over the past five years, there has been a very significant fracture. As soon as Zelensky tries to overcome it, he will face resistance from radicals, nationalists and other political opponents," Oktysyuk said.

U.S. President Donald Trump spoke with Zelenskiy Sunday to congratulate him on his victory.

A statement from the White House said Trump reaffirmed the U.S. support for Ukraine’s sovereignty and territorial integrity within its internationally recognized borders. The U.S. president expressed his commitment to work with Zelenskiy and the Ukrainian people to implement reforms needed to strengthen democracy, increase prosperity and root out corruption in the country.

Poroshenko is expected to hand over power next month, giving the comedian who played the president on TV a shot at the real thing. It is a big step into the political unknown and an indication of how desperate Ukrainians are for change.

VOA.