Comedian Volodymyr Zelensky has been sworn in as Ukraine’s new president at a ceremony in the capital Kiev.
In his inaugural speech, he said ending the conflict with Russian-backed rebels in the east would be his top priority.
Mr Zelensky, who has no previous political experience, scored a landslide victory in last month’s presidential election.
“We must become Icelanders in football, Israelis in defending our land, Japanese in technology,” he said.
Hinting at Russia, without naming it, he said Ukrainians must also “become Swiss in our ability to live happily with each other, despite any differences”.
“Our first task is to achieve a ceasefire in Donbass,” he said, referring to the eastern region controlled by Russian-backed rebels.
In the election the former television actor ousted incumbent Petro Poroshenko, who had been in power since 2014.
Mr Zelensky has promised to tackle corruption, but has given few details of his plans since winning by a landslide on 21 April.
At the ceremony, Mr Zelensky was given golden symbols of office, including a sceptre, which he held aloft in a victory salute.
A choir in folk costume sang patriotic hymns to welcome him.
Russian state TV said no Russian official had been invited to the inauguration.
Mr Zelensky is delivering his inaugural address in Ukrainian.
But at one point, referring to the conflict in the east, he switched to Russian, saying: “I’m convinced that for this dialogue to start, we must see the return of all Ukrainian prisoners” – apparently a message to the Russian government.
There has been much political wrangling over the timing of Mr Zelensky’s inauguration. He had wanted to hold it on Sunday, a day of mourning for victims of Stalin-era repression in the country.
But parliament disagreed and voted to hold it on Monday morning.
From fiction to reality
Analysis by Jonah Fisher, BBC News, Kiev
Voldymyr Zelensky has done this all before – albeit in a TV series, where he played the part of Ukraine’s president.
At that fictional inauguration he arrived in a taxi and, in his speech, refused to make bold promises. Instead he pledged to do his best for the people of Ukraine.
On Monday it’s happening for real. And it may not be that different.
Mr Zelensky is unlikely to turn up for this inauguration in a taxi but he has – via a video on social media – already apologised to the people of Kiev for the traffic congestion the event is likely to cause.
No one is quite sure what his inaugural speech will contain.
During the month since winning the election Mr Zelensky has given no press conferences or interviews. He has left it to a team of advisers to try and reassure people that he knows what he’s doing.
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 from the other candidates rather than on any concrete policy ideas.
Despite this, he stormed to victory and won more than 70% of the vote.
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
Source: Ukrainian and international media
What challenges will he face?
Mr Zelensky is likely to be confronted with a range of serious issues once he takes office.
He made tackling Ukraine’s deep-rooted corruption a major campaign theme. Last month, his aides announced plans to scrap MPs’ immunity from prosecution and make military purchases more transparent.
But perhaps the biggest challenges the new leader will face is the continuing conflict with Russian-backed forces in the east.
In the run-up to his election, Mr Zelensky said he wanted to “renew relations” with eastern Ukraine and start a “powerful information war to end the conflict”.
Fighting in the region has claimed about 13,000 lives since Moscow annexed Ukraine’s Crimea peninsula in 2014.
There have already been indications that Russian President Vladimir Putin is determined to test the political novice.
Shortly after the election, Mr Putin made it easier for those living in eastern Ukraine’s separatist territories to obtain a Russian passport. The move was widely seen as a challenge to Mr Zelensky.
In a Facebook post in response, Mr Zelensky’s team labelled Russia “an aggressor state which wages war against Ukraine”.
It called on the international community to provide “diplomatic pressure and the pressure of sanctions”.