George Papandreou has been sworn in as Greece's prime minister after his Panhellenic Socialist Movement (Pasok) won general elections on Sunday.
The son and grandson of former prime ministers was sworn in at a ceremony in the president's Athens mansion.
Mr Papandreou, 57, is to announce his cabinet appointments later.
Pasok won 44% of votes in the snap polls, ending five years of governance by the conservative New Democracy party of Costas Karamanlis, who has resigned.
Mr Karamanlis' party won just 33.5% of vote - its worst ever election result.
Having won an absolute majority with 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 after the results were announced.
"I will always be upfront with the Greek people, so we can solve the country's problems together."
Mr Karamanlis called the snap general election half way through his four-year term.
He said he wanted a new mandate to tackle Greece's economic problems, but his opponents said he failed to fulfil promises to clean up public office and to modernise the country.
The government has also been hit by a series of corruption scandals.