Supporters of radical leftist Syriza party chant slogans and wave Greek national and other flags after winning elections in Athens, January 25, 2015.[Photo/Agencies]

 Thousands of supporters turned out to watch the 40-year-old Tsipras speak in central Athens after his opponents conceded.

"The Greek people have written history," he said, to cheers. "Greece is leaving behind catastrophic austerity, fear and autocratic government."

Outside the party's campaign tent in central Athens, supporters hugged each other and danced in celebration.

"It's like we've been born again and finally feel some hope," said Litsa Zarkada, a fired government cleaning worker. "We were thrown into the street just before we could take our pension. We have been through so much.

The new government faces an immediate cash shortage, with a dwindling primary surplus, upcoming loan repayments, and limits on the money it can raise using treasury bill auctions.

Megan Greene, chief economist at Manulife Asset Management, said the government will be unable to afford to run its day-to-day operations and pay back debt that falls due in March in the absence of additional cash from international creditors.

"Syriza and its creditors are stuck in a Gordian Knot, and both sides will need to cave on something. Neither Greece nor its creditors want Greece to default or exit the eurozone, so a compromise will probably be found," Greene told the AP.

"If Syriza forms a coalition with the Independent Greeks, that suggests the new government will engage in dangerous brinkmanship with Greece's creditors as it tries to negotiate funding to stave off utter bankruptcy over the next few months."