Alexis Tsipras has been sworn in as Greece's youngest prime minister in 150 years and is set to lead an anti-austerity coalition after an election win that has sent shockwaves through Europe.
Tsipras's decisive victory in Sunday's vote set the country on a collision course with international creditors over plans to renegotiate its massive bailout deal.
His radical-left Syriza took 149 of the 300 seats in parliament, becoming the first party to take power in Europe that openly rejects spending cuts and austerity measures.
Having fallen two seats short of an absolute majority, the leftists quickly forged a coalition with the small right-wing Independent Greeks (ANEL) party.
The new cabinet is expected to be announced and sworn in on Tuesday.
At his swearing-in ceremony, Tsipras broke with tradition by taking a civil instead of a religious oath.
His first priority will be to deliver on his pledge to renegotiate the terms of the country's 240 billion-euro EU-IMF bailout despite universal opposition from Greece's European allies.
Renewed fears that Greece could be forced out of the eurozone if it defaults on its debt repayments led to the euro hitting an 11-year low against the US dollar while Greek stocks closed down more than three per cent.
The IMF extended an olive branch to the new Greek government on Monday, saying it was prepared to continue its financial support to the country.
"We stand ready to continue supporting Greece, and look forward to discussions with the new government," IMF managing director Christine Lagarde said in a statement.
However, EU Commission chief Jean-Claude Juncker warned that Greece could not expect any reduction of its huge debt commitments.
"There is no urgent need for action" on Greece's debt, Juncker told German television station ARD, adding that a reduction of the debt "is not on the radar".
Tsipras said on Sunday his victory meant Greece was "leaving behind disastrous austerity" and was a "mandate for national rebirth" after the "humiliation" of years of painful austerity.
Many Greeks were optimistic that the fortunes of a country mired in deep recession for six years were about to change for the better.
Nikos, a Syriza supporter in Athens, said: "Today is a very good day. I believe things will go well for our country."
But others were sceptical.
"There are many promises, but at the end there will be nothing. They only want power," Athina Mantsinou said.
Thousands of people had poured into the streets of Athens on Sunday, partying through the night after Syriza's bigger-than-expected win.
But in a sign of the mammoth challenge ahead, the EU issued a stern statement that Greece risked its place in the eurozone if it failed to meet its austerity and debt commitments.
From Brussels to Berlin, officials said they were open to talks with the new team in Athens, but many signalled its proposals were unrealistic.
In exchange for the bailout in 2010, Greece was forced to slash public sector spending, cut wages and pensions and introduce a far-reaching program of privatisation.
Syriza has pledged to reverse many of those measures.
But Chancellor Angela Merkel of European paymaster Germany made her views clear.
"In our view it is important for the new government to take action to foster Greece's continued economic recovery," her spokesman Steffen Seibert said. "That also means Greece sticking to its previous commitments."
Analysts said the eurozone was set for fresh turbulence, and some noted the coalition between Syriza and the Independent Greeks could prove short-lived.
"A period of uncertainty and heightened market nervousness now seems likely," Jonathan Loynes of Capital Economics said.
Sunday's poll was Greece's fourth in five turbulent years, including back-to-back votes in 2012.
During that time, the economy has shrunk by a quarter and unemployment has soared beyond 25 per cent.
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