Ukrainian comedian Volodymyr Zelensky has scored a landslide victory in the country's presidential election.
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 (in Russian).
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.
73%landslide win on 21 April
Comedian but trained as a lawyer
Instagram star with 4.2m followers
Millionaire thanks to his production company Kvartal 95
Linked to oligarch Ihor Kolomoisky through show on TV channel 1+1
He told reporters he would "reboot" peace talks with the separatists fighting Ukrainian forces and volunteers in the east.
"I think that we will have personnel changes. In any case we will continue in the direction of the Minsk [peace] talks and head towards concluding a ceasefire," he said.
There are sporadic skirmishes and the situation also remains tense around Crimea, annexed from Ukraine by Russia in 2014.
Time for the joking to stop
Analysis by Jonah Fisher, BBC News, Kiev
Ukrainians are waking up this morning and discovering that the last few months were not a dream.
They really have elected a man who currently stars in a TV series as the president - as the country's next real president. And it wasn't even close.
The pressure will now be on Mr Zelensky to demonstrate that he knows what he is doing.
Throughout the election campaign, he avoided serious interviews and discussions about policy - preferring instead to post light-hearted videos to social media.
He's got about a month before the inauguration. Then the comedian-turned-president will be faced with a complex in-tray that includes a simmering war with Russian-backed rebels in the east.
Mr Poroshenko, who has been in power since 2014, said the result of the election "leaves us with uncertainty [and] unpredictability".
"I will leave office but I want to firmly stress - I will not quit politics," he said.
Billionaire Mr Poroshenko was elected after an uprising overthrew the country's previous pro-Russian government.
In a tweet, he said "a new inexperienced Ukrainian president... could be quickly returned to Russia's orbit of influence".
But Russia's foreign ministry said Ukrainian voters had expressed their desire for political change.
"The new leadership now must understand and realise the hopes of its electors," deputy foreign minister Grigory Karasin told the Ria Novosti news agency. "This of course applies to domestic as well as foreign affairs."
What's the reaction been?
French President Emmanuel Macron congratulated Mr Zelensky in a phone call, as did Poland's President Andrzej Duda.
In a tweet, UK Foreign Secretary Jeremy Hunt said: "You will now truly be the Servant of the People."
The US embassy in Ukraine tweeted its congratulations while European Council President Donald Tusk said the EU was "determined to continue its support" for Ukraine.
A decisive day for Ukraine. Free elections and peaceful change of power = strong Ukrainian democracy. Congratulations to Volodymyr Zelensky. The EU is determined to continue its support.— Donald Tusk (@eucopresident) April 21, 2019
Who is Volodymyr Zelensky?
Mr Zelensky starred in the long-running satirical drama Servant of the People in which his character accidentally becomes Ukraine's president.
He plays a teacher who is elected after his expletive-laden rant about corruption goes viral on social media.
He ran under a political party with the same name as his show.
With no previous political experience, Mr Zelensky's campaign focused on his difference to the other candidates rather than on any concrete policy ideas.
Despite this, he won the first round with more than 30% of the vote - almost double what Mr Poroshenko got when he finished in second place with 15.95%.
What do voters think of him?
Analysts believe Mr Zelensky's informal style and vow to clean up Ukrainian politics resonated with voters who are disillusioned with the country's path under Mr Poroshenko.
Eschewing traditional campaign tactics, Mr Zelensky channelled his on-screen persona by promising to stamp out corruption and loosen the grip of oligarchs on Ukraine.
Experts say his supporters, frustrated with establishment politicians and cronyism, have been energised by his charisma and anti-corruption message.
His critics, meanwhile, are sceptical about his credentials, with many expressing concern over his close links to the billionaire oligarch Ihor Kolomoisky.
They have expressed doubts that he will be able to take on the country's influential oligarchs and stand up to Russian President Vladimir Putin.