ATHENS - Greece's conservative New Democracy party stormed to victory in a parliamentary election on Sunday with voters giving reformist Kyriakos Mitsotakis another four-year term as prime minister.

With most votes counted, centre-right New Democracy was leading with 40.5% of the vote and 158 seats in the 300-seat parliament, interior ministry figures showed.

It was more than 20 points clear of Syriza, a radical leftist party that won elections in 2015 at the peak of a debilitating debt crisis and ran the country until 2019, when it lost to New Democracy.

"This freely given support only increases my responsibility to respond to peoples' hopes. I personally feel an even stronger obligation to serve the country with all my abilities," Mitsotakis told cheering crowds at New Democracy headquarters in downtown Athens.

Sunday's vote was a humiliating defeat for Syriza, which lost more than 30 MPs. Fringe parties of the political left and right - including an anti-immigrant party calling themselves the Spartans - got a foothold in parliament.

"This result is negative for democracy and society," Tsipras said, referring to far-right parties winning votes. For Syriza, he said, a "great and creative historic circle had closed."

"We have to look upon that with pride," he said.

Mitsotakis, 55, a former banker and scion of a powerful political family, has promised to boost revenue from the vital tourist industry, create jobs and increase wages to near the European Union average.

Mitsotakis, who was prime minister from 2019 until stepping down in favour of a caretaker premier following an inconclusive May vote, has vowed to push ahead with reforms to rebuild the country's credit rating after the debt crisis that wracked the nation for a decade.

Sunday's vote was the second in the past five weeks, as a first poll on May 21 held under a different electoral system failed to give a single party absolute majority in parliament. The system used in Sunday's poll gave the leading party bonus seats depending on voter support.

Zoe Constantopoulou, a leftist politician who spent hours regaling Greece's lenders at the height of the country's debt crisis in 2015 when she was parliamentary speaker, saw her party, Plefsi Eleftherias, gain 8 seats in parliament.

"Whether we are eight or nine MPs, I'm good enough for 100 (MPs)," she said.

The COVID-19 pandemic and a deadly rail crash in February exposed shortcomings in Greece's health and public transport systems. But a cost-of-living crisis and economic hardship have more recently topped voters' concerns.

"I expect a lot (from the new government)," pensioner Giorgos Katzimertzis told Reuters.

"The main thing is the health system, the economy, so we can live (decently) because things are difficult. I am a pensioner, I was on the fire brigade, and now I don’t have a dime."

Sunday's election was held in the shadow of a migrant shipwreck this month in which hundreds are feared to have perished off southern Greece. One of the worst such disasters in years, it has exposed the parties' divisions over migration.

The Spartans party, which said Greece was threatened by uncontrolled migration, was the surprise of the campaign. It was set to gain 4.7 of the vote and up to 13 seats in parliament, based on the early results.

The group was catapulted from relative obscurity after support from Ilias Kasiadiaris, the frontman of the now-banned Golden Dawn far-right party. His own party was barred from the elections and he endorsed the Spartans from jail.

A Twitter post from Kasidiaris showed him beaming, wearing a t-shirt with a Spartans logo and giving the thumbs up, super-imposed against jail bars.

(Reporting by Gina Kalovyrna, Michele Kambas, Renee Maltezou, Lefteris Papadimas and Angeliki Koutantou; Writing by Michele Kambas; Editing by Frances Kerry, Emelia Sithole-Matarise, Giles Elgood and Mark Porter)