With 85 per cent of constituency results counted, the Socialist party had achieved more than 43 per cent of the vote.
The results indicate that Antonio Guterres will become the first Socialist leader in a decade to become Portugal's prime minister. A senior Socialist spokesman, Jaime Gama, declared last night that his party had won "a clear mandate". He added: "We will
Shortly afterwards, the outgoing conservative Social Democrat, Anibal Cavaco Silva, conceded defeat. "The PS is the de facto victor in these elections," he said.
The results indicated that the Socialists would have an absolute majority of one or two MPs, some 9 per cent ahead of their nearest rivals, the Social Democrats. The right-wing Popular Party headed for third place, with around 9 per cent, a considerable advance on its previous vote, with the Communist-led CDU alliance expected to win some 8.5 per cent.
Mr Guterres, a pragmatic politician who promised the cautious Portuguese "change without trauma", has said that he will try to form a government without formal alliances with other parties.
His programme differs little in substance from that carried out by Mr Cavaco. Both share a commitment to being among the EU countries adopting a single currency, and both favour a policy of privatisations, softened during the persistent economic recession by social reform.
But the PS promises a more open government, and to bring new faces and enthusiasm to the political scene.