Greek Prime Minister Alexis Tsipras has stormed back into office with an unexpectedly decisive election victory.

His leftist Syriza party won 35.47 percent of the vote in yesterday's Greek national election based on 99.44 percent of votes counted, the Interior Ministry said on its website.

Conservative New Democracy won 28.09 percent.

That share gives Syriza 145 seats in the 300-seat parliament and New Democracy 75. The turnout was 56.5 percent.

Turnout in the January election won by Tsipras was 63.6 percent.

The Greek leader says the wide margin of victory gives him a clear mandate to steer Greece's battered economy to recovery.

"Today in Europe, Greece and the Greek people are synonymous with resistance and dignity. This struggle will be continued together for a full four years, because the mandate we got is a four-year mandate," Tsipras said.

The win comes just one month after Tsipras was abandoned by party radicals who accused him of caving in to demands for austerity in order to secure a financial bailout from European leaders.

The bailout programme is due for a review next month. Tsipras is hoping to persuade lenders that enough of the austerity programme has been implemented to ensure the next payment.

He will also have to grapple with Greece's central role in Europe's refugee crisis, as the main entry point for tens of thousands of migrants.

He'll meet EU colleagues at an emergency summit on the issue later this week.