Outgoing Prime Minister Francisco Pinto Balsemao quit the leadership of his Social Democratic Party today after suffering an overwhelming defeat to conservative hard-liners in a party congress here.

In a key vote, Portugal's major government party voted to replace Balsemao's post as party president with a ruling triumvirate.

Balsemao, who became party leader and prime minister in December 1980 when his charismatic predecessor, Francisco sa Carneiro, was killed in an airplane crash, said he would remain in the party at a grass-roots level but would not seek reelection to the leadership.

The congress decision represents a victory for the party's conservative faction whose bitter attacks on Balsemao's moderate leadership forced him to resign as premier in December.

The vote means the trio of leaders to emerge from the congress Sunday will almost certainly be headed by conservative former prime minister Carlos Mota Pinto, a university professor, according to observers here.

He is expected to attempt to revive the party's crumbling government alliance with the smaller rightist Christian Democrats and steer away from a possible post-election coalition with the opposition Socialists.

The Socialists, led by former prime minister Mario Soares, are widely expected to return as the biggest single party after an early parliamentary election called for April 25 to end the drawn-out government crisis provoked by Balsemao's resignation.