Comedian Volodymyr Zelensky announced a snap parliamentary election during his swearing-in as Ukraine's new president.
The election was expected to take place in October 2019. But at the ceremony in Kiev Mr Zelensky said "I am dissolving the Verkhovna Rada (parliament)".
He said ending the conflict with Russian-backed rebels in the east would be his top priority.
A political novice, he campaigned strongly against official corruption.
"People must come to power who will serve the public," he said on Monday.
Russian President Vladimir Putin said he would not congratulate Mr Zelensky on his inauguration, but would wait for "the first successes in settling the internal conflict in south-eastern Ukraine, and in normalising Russian-Ukrainian relations".
Meanwhile, Russia's Foreign Minister Sergei Lavrov called on Ukraine's new leader to implement the Minsk peace accord agreed in 2015, and also suggested an "all for all" prisoner swap between both sides.
Mr Zelensky a former television star, acted the part of Ukrainian president in a popular comedy series. He scored a landslide victory in last month's presidential election.
"We must become Icelanders in football, Israelis in defending our native land, Japanese in technology," he said at the swearing-in.
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 Donbas," he said, referring to the eastern region controlled by Russian-backed rebels.
In the election Mr Zelensky ousted incumbent Petro Poroshenko, who had been in power since 2014.
Mr Zelensky has given few details of his plans since winning by a landslide on 21 April. He has left it to a team of advisers to try and reassure people that he knows what he is doing.
At the ceremony, Mr Zelensky was given golden symbols of office, including a sceptre, which he held aloft in a victory salute.
Russian state TV said no Russian official had been invited to the inauguration.
Mr Zelensky made 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".
Fighting in the east has claimed about 13,000 lives since Moscow annexed Ukraine's Crimea peninsula in 2014.
Putting on a show
Analysis by Jonah Fisher, Kiev Correspondent
Ukraine saw the Volodymyr Zelensky they knew so well. With a broad smile on his face, he strode the last few hundred metres to the Verkhovna Rada, high five-ing supporters and posing for selfies.
Once inside, things got serious. Surrounded by the politicians he has spent most of his career mocking, Mr Zelensky was sworn in and made his inaugural address.
As far as I can tell, it was the first full-length public political speech the former comedian has made, and he pulled no punches.
During a passage about the conflict in eastern Ukraine he alternated between the Russian and Ukrainian languages, drawing a heckle from an MP, followed by a withering put-down from Mr Zelensky: "Thanks for continuing to divide the nation, Mr Lyashko."
Mr Zelensky accused Ukraine's politicians of "creating a country of opportunities - opportunities to steal, bribe and loot", before announcing that he was dissolving parliament and triggering new elections.
If the new president is to have a chance of delivering on his ambitious plans, he will need the people who swept him to power to transform parliament too.
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 73% of the vote.
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
What challenges will he face?
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 the eastern rebels.
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".
There have already been indications that President Putin is determined to test him.
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".