The comedian who previously played the president in a TV show has announced he would dissolve parliament in order to call an early election.
Ukraine's new President Volodymyr Zelensky called snap parliamentary polls in his inaugural speech Monday and said his top priority is ending the war with Russia-backed separatists in the country's east.
The 41-year-old comedian was sworn in as Ukraine's youngest post-Soviet president a month after scoring a landslide victory over Petro Poroshenko with a campaign capitalising on widespread discontent with the political establishment amid poverty and corruption.
Mr Zelensky - whose only previous political experience was appearing as president in a popular TV show - announced he would dissolve parliament in order to call an early election, originally scheduled for October.
"People must come to power who will serve the public," Ukraine's sixth president said, after wrangling with hostile lawmakers whom he called "petty crooks".
Prime Minister Volodymyr Groysman said he would be resigning on Wednesday.
Mr Groysman said he had offered to work under Mr Zelensky but added that "the president chose a different path."
The legal status of Mr Zelensky's move to dissolve parliament is uncertain but it is still likely to go ahead, political analysts said.
"There are no mechanisms or instruments to stop this decision," said analyst Mykola Davydchuk.
'Ready for dialogue'
Mr Zelensky said in his speech in Kiev that "our first task is a ceasefire in the Donbass," referring to the eastern separatist-controlled region, prompting a round of applause.
"We didn't start this war but it is up to us to end it," he said.
"We are ready for dialogue," he added, urging the handover of Ukrainian prisoners.
Kremlin spokesman Dmitry Peskov said Russian President Vladimir Putin has no plans to meet Mr Zelensky and would not be congratulating him on his inauguration.
The Russian president will only "congratulate him on the first successes" in resolving the separatist conflict, he said, calling it a "domestic problem" for Ukraine.
Kiev and its allies accuse Moscow of militarily supporting the separatists, which it denies.
The conflict - which broke out after Russia annexed Crimea - has claimed some 13,000 lives since 2014.
Russian Foreign Minister Sergei Lavrov said that an exchange should include all prisoners from both sides.
Mr Zelensky said Ukraine's "next challenge is the return of the lost territories," referring to Moscow-annexed Crimea and the separatists' self-proclaimed republics.
Switching to Russian in an emotive speech, he stressed that Ukraine must regain the trust of Russian-speaking people living there, who "are not strangers, they are ours, Ukrainians."
Zelensky also called for the sacking of the head of the state security service, prosecutor-general and defence minister loyal to his predecessor, although this has to be approved by parliament.
Defence Minister Stepan Poltorak and the head of the SBU security service Vasyl Grytsak tendered their resignations on Monday. Foreign minister Pavlo Klimkin stepped aside last week.
'Not an idol'
Mr Zelensky took a non-traditional route to his inauguration - walking from his nearby home, after saying he wanted a less pompous ceremony.
Dressed in a dark suit, he exchanged high fives with supporters waiting outside, took selfies with them and even jumped up to plant a kiss on a supporter's forehead.
In his speech, Mr Zelensky referred to his background as a comedian. "In my life, I've tried to do all I could to make Ukrainians smile," he said.
"In the next five years I'll do all I can so that Ukrainians don't cry."
He urged officials to hang pictures of their children in their offices -- and not his photograph.
"A president is not an idol," he said.
The story of his rise mirrored that of his character in the hit sitcom, "Servant of the People."
He starred as a history teacher who was unexpectedly elected president after his expletive-laden rant about corruption went viral.
His newly-formed party, named "Servant of the People", is already leading in opinion polls.
When the actor and comedian announced his candidacy on New Year's Eve, few took it seriously, but after a campaign largely waged through social media, he won more than 73 per cent in the second round on 21 April against Mr Poroshenko.
Mr Poroshenko led Ukraine for five years, grappling with the fallout from Russia's 2014 Crimea annexation and the war in the east.
Although he pushed through some reforms, he was criticised for failing to improve living standards or root out corruption.
Mr Zelensky has vowed to press ahead with the country's pro-Western course but critics question how he will deal with the enormous challenges of the separatist conflict and ongoing economic problems.
After Mr Zelensky's landslide victory, Mr Putin ordered an easing of procedures for millions of Ukrainians to gain Russian passports, a move that caused uproar in Kiev.
Kiev's Western partners said they were ready to support the new leader and in a first concrete move led by the United States they blocked a Russian request to hold a UN Security Council meeting on Ukraine's new language law.
Last month, Ukraine's parliament passed the law, which enforces use of the Ukrainian language in official settings, a move strongly denounced by Russia.
"The United States looks forward to partnering closely" with Mr Zelensky, the US embassy in Kiev said on Twitter.
Amelie de Montchalin, France's European Affairs minister, said Paris will "support and carefully watch" the reforms announced by Mr Zelensky.