Greek Prime Minister Alexis Tsipras called snap national elections after a severe defeat by the opposition conservatives in the European Parliament elections on Sunday.
Provisional results showed ruling party Syriza trailing the opposition New Democracy party by about ten points.
"I will request immediate declaration of national elections from the President of the Republic," Tsipras said in a speech.
The government's current term was due to expire in October.
New Democracy leader Kyriakos Mitsotakis demanded Tsipras resign, saying he had lost the popular mandate.
"The Prime Minister must assume his responsibilities. For the good of the country, he must resign and the country should hold national elections, the soonest possible," Mitsotakis said.
A party source said the earliest a vote could take place would be June 30, to allow for preparations.
Why Syriza lost steam
Syriza stormed the Greek political scene on an anti-austerity platform six years ago, then suffered a backlash after having to impose cut-backs as part of a third bailout in 2015.
Greece emerged from close financial supervision by its lenders in August 2018. The government this month introduced tax cuts and pension payouts, unwinding some of the austerity measures.
The handouts may have averted a steeper defeat in the European election, political analyst Thanos Veremis said.
More damagingly, Tsipras signed a deeply unpopular agreement that resolved a long-running name dispute with North Macedonia in June.
Tsipras' coalition partner, Panos Kammenos, pulled out of the government in January, triggering a confidence vote in parliament that the Prime Minister won comfortably.
Tsipras signed the name-change deal on the banks of the Prespes Lake bordering Albania, Greece and North Macedonia. It officially came into effect in February. For most, use of the Macedonia name is an appropriation of Greek heritage by the country's northern neighbour.
"Syriza committed suicide in Prespes," Kammenos, his former coalition partner, tweeted on Sunday evening.