Alexis Tsipras, the one-time leftist firebrand who stormed to power on an anti-austerity agenda in Greece in 2015, stepped down from the helm of the Syriza party after 15 years on Thursday, following a heavy election defeat.

At the height of Greece's deep economic crisis, Tsipras, known for sporting his no-tie look, rode a wave of anti-establishment anger to become the country's first leftist prime minister.

Tsipras, and his finance minister at the time Yanis Varoufakis, were global household names when Greece was racked with violent street protests against the international bailouts that saved the country from bankruptcy but imposed painful austerity on Greeks.

Syriza lost to Prime Minister Kyriakos Mitsotakis's New Democracy in 2019, and in Sunday's vote, won just 17.8% of the votes, against 40.5 for New Democracy.

"The time has come to start a new cycle," Tsipras said in a televised address announcing his resignation.

"The negative election result can – and must – become the beginning of this cycle," he said.

Tsipras said he would not be a candidate at elections for a new party leader.

Tsipras' decision to step down was "sudden", a party source said, but also "the right one for the party's reform and for his own legacy."

When elected at the age of 40, Tsirpas, an engineer by training and a former member of the Communist youth, was one of Greece's youngest-ever prime ministers.

Soon after taking power, he railed against austerity and the bailout programmes backed by the European Union and the International Monetary Fund that he says had triggered a humanitarian crisis in Greece.

But after months of fierce fighting with other euro zone countries, Tsipras was forced in June 2015 to impose capital controls. In July, he won a referendum urging Greeks to reject an offered bailout, but made a U-turn and signed up to a tougher program a month later.

Since Greece's third and final bailout programme ended in 2018, the country has regained market access, cut down its record debt and growth is set to outpace the euro zone's average. Public anger has quietened down as a result.

Following an inconclusive general election in May, in which Syriza came second, political analysts said Syriza missed the willingness of the electorate to move away from the crisis years.

"I am proud of everything that happened," Tsipras said on Thursday. "This difficult journey had compromises, and difficult decisions, and injuries and attrition, but it was a journey that left a mark on history."