Socialist leader George Papandreou joins party celebrations
Greece's opposition Panhellenic Socialist Movement (Pasok) has won an emphatic general election victory.
Outgoing PM Costas Karamanlis has congratulated Pasok leader George Papandreou and resigned as leader of the conservative New Democracy party.
With 99% of votes counted, Pasok had nearly 44% of the vote, to 33.5% for New Democracy - its worst ever result.
Pasok needed 43% to win an absolute majority in parliament. It has been in opposition for more than five years.
With a projected 160 seats in the country's 300-member parliament, Mr Papandreou has said he will inject up to 3bn euros ($4.4bn: £2.7bn) into the economy in an effort to pull Greece out of a financial crisis.
"Nothing is going to be easy. It will take a lot of hard work," he said. "I will always be upfront with the Greek people, so we can solve the country's problems together.
US President Barack Obama was among the first international leaders to congratulate Mr Papandreou in a telephone call on Sunday night.
'Set free potential'
In his first speech after being elected, Mr Papandreou warned Greeks they faced tough times.
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