Alexis Tsipras was sworn in Monday as Greek prime minister at the helm of an anti-austerity coalition after an election win that has sent shockwaves through Europe.
Tsipras's radical leftwing Syriza party cruised to victory in Sunday's vote, setting the country on a collision course with international creditors over plans to renegotiate its massive bailout deal.
Syriza took 149 out of the 300 seats in parliament, becoming the first party to take power in Europe that openly rejects spending cuts and austerity measures.
Tsipras -- characteristically wearing an open-necked shirt without a tie at his investiture -- also broke with tradition by taking a civil instead of a religious oath.
The 40-year-old former Communist youth activist pledged to "always serve the interests of the Greek people" as he became the country's youngest prime minister for 150 years.
Having fallen two seats short of an absolute majority, Syriza quickly forged a coalition with the small right-wing Independent Greeks (ANEL) party.
Tsipras' first priority will be to deliver on his pledge to renegotiate the terms of the country's massive 240-billion-euro ($269 billion) 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 saw the euro hit an 11-year low against the dollar while Greek stocks closed down more than three percent.
Tsipras said Sunday his victory would see Greece "leaving behind disastrous austerity".
Syriza mocked claims by former prime minister Antonis Samaras' New Democracy party that the country would grind to a halt if it elected the radical left-wing party.
"The sun is shining, the bank cash dispensers are working and people are at work," said party spokesman Panos Skourletis.
- 'Change for better' -
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 other Greeks were sceptical.
"There are many promises, but at the end there will be nothing. They only want power," said Athina Mantsinou as she walked through the capital's Syntagma Square -- scene of many demonstrations against austerity.
Thousands of people had poured into the streets of Athens on Sunday, partying through the night after Syriza's bigger-than-expected win.
As supporters waved the party's red and white flag, Tsipras said Syriza's victory was a "mandate for national rebirth" after the "humiliation" of years of painful austerity.
But in a sign of the mammoth challenge ahead, the EU issued a stern warning to Greece that its place in the eurozone would be at risk 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 programme 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 that the coalition between Syriza and the Independent Greeks could prove short-lived.
"A period of uncertainty and heightened market nervousness now seems likely," said Jonathan Loynes of Capital Economics.
Europe's top selling newspaper, Germany's Bild, called Syriza's platform "dangerous" for trying to reverse the EU-led economic reforms.
"Sorry, Mr Tsipras, but that goes too far!" Bild declared.
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 percent.
Samaras had pleaded with voters not to abandon the painful public sector spending cuts, saying they were helping Greece emerge from recession.
But his New Democracy party was routed and reduced to 76 seats.
- Inspiration to others -
Syriza's victory could inspire other anti-austerity parties in Europe, including Spain's Podemos, which has topped several opinion polls and is aiming for an absolute majority in the Spanish election in November.
"Our victory is also a victory for all European peoples fighting against austerity that is destroying our common European future," Tsipras told supporters Sunday.
Podemos leader Pablo Iglesias said: "The Greeks are going to have a true Greek president, not a delegate of... Merkel."
British finance minister George Osborne -- whose country does not use the euro -- said what Syriza was proposing was "incompatible with what the eurozone currently demands of its members."
However some European countries struck a warmer tone, saying they were prepared to work with the new Greek government.
French President Francois Hollande invited Tsipras to visit and Spain's conservative Prime Minister Mariano Rajoy said he hoped the victory would lead to a "stable government".
Italy saw the Greek result as possibly helping its push for greater flexibility in the EU's approach to budget and broader economic issues, Foreign Minister Paolo Gentiloni said.
"There has been a tug of war for months between austerity and flexibility," Gentiloni told Italian TV.