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Alexis Tsipras on Thursday said he will step down as leader of Greece's left-wing Syriza opposition party following a crushing defeat at the polls.

His announcement came after voters handed his conservative rival, New Democracy leader Kyriakos Mitsotakis, an imposing election victory for the second time in five weeks on SundayÌý– while Syriza saw a loss of tens of thousands of voters.

The conservatives landed their widest winning margin in almost 50 years, with Mitsotakis securing a second term as Greece's prime minister.

"There are times when crucial decisions have to be taken," an emotional Tsipras told the press in Athens after a meeting of Syriza's executive.

He said he would call "elections within Syriza for a new leader, in which I will not be a candidate".

"A chapter is closing and a new one must be opened for our party," he said, adding that Syriza "needs renewal and major reforms".

For many Greeks, former premier Tsipras is the prime minister who nearly crashed Greece out of the euro, and who reneged on a vow of abolishing austerity to sign the country on to more painful bailout terms.

Syriza received just under 18 percent of the vote, more than 20 points behind New Democracy.

After Mitsotakis's victory, Tsipras acknowledged a "serious political defeat" and said he was leaving his fate to the "judgement" of Syriza members.

"I'm used to not taking decisions in the heat of the moment," the 48-year-old said on Thursday.

"I took the time to reflect for three days to make a cool-headed decision."