Greece`s opposition Panhellenic Socialist Movement (Pasok) has won the country`s snap general elections, according to BBC.
Outgoing PM Costas Karamanlis has congratulated Pasok leader George Papandreou and resigned as leader of the conservative New Democracy party.
With most ballots counted, the Socialists had more than 43% of the vote, to 35% for New Democracy.
Pasok needs 43% to win an absolute majority in parliament. It has been in opposition for more than five years.
Mr Papandreou, 57, told cheering supporters in Athens: "We stand here united before the great responsibility which we undertake."
He said Pasok had waged "a good fight to bring back hope and smile on Greeks` faces... to change the country`s course into one of law, justice, solidarity, green development and progress".
He added: "I know very well the great potential of this country. Potential that is being drowned by corruption, favouritism, lawlessness and waste. Potential that we will set free.
"I promise that I will do whatever is possible so that all Greeks will believe again that we can succeed, when we are united."
Earlier, in a televised address, a humbled Mr Karamanlis said: "I assume responsibility for the result and will launch procedures for the election of a new party leader."
Mr Karamanlis called the election in early September, half way through his four-year term.
He said he wanted a new mandate to tackle Greece`s economic problems, but his opponents say he has 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.
Mr Papandreou has promised he will build a green economy and bring in foreign experts to help Greece overcome its problems if elected.
The BBC`s Malcolm Brabant says voters preferred Mr Papandreou`s promise of a 3bn Euro ($4.4bn:£2.7bn) stimulus package to the programme of austerity proposed by Mr Karamanlis.
Correspondents say the build-up to the election has been lacklustre. A recent poll has also suggested nine out of 10 voters no longer trust either party.
On Friday a small bomb exploded near Mr Karamanlis`s final campaign rally.
The blast, which caused no injuries and only minor damage, was claimed by a leftist group calling itself The Fire Conspiracy Cells the following day.
Forty seats are automatically awarded to the leading party and the remaining 260 are divided by proportional representation.