Greek Socialists trounced the governing conservatives in a landslide election, with voters angered by scandals and a faltering economy ousting Prime Minister Costas Karamanlis halfway through his second term.
With a partial vote count showing that his New Democracy Party was heading for its worst electoral performance ever, Karamanlis, 53, resigned as its leader and said a new chief is needed for the party founded 35 years ago by his late uncle Constantine Karamanlis.
George Papandreou, 57, now follows in the footsteps of his father, Andreas Papandreou, who founded his Panhellenic Socialist Movement party, and grandfather and namesake George Papandreou, both of whom served several terms as prime ministers.
"We bear a great responsibility to change the course of the country. . . . We know that we can make it," Papandreou, a former foreign minister, told jubilant supporters outside his party headquarters in central Athens.
Results from 60% of votes counted showed the Panhellenic Socialist Movement winning with 43%, compared with 34% for New Democracy.