President Constantine Karamanlis formally asked a former U.S. economics professor today to form a new government, setting the stage for this country's first socialist government.

Andreas Papandreou, 62, the leader of the Pan-Hellenic Socialist Movement (Pasok) that triumphed by a landslide in national elections here Sunday, received the presidential mandate during a private meeting with the head of state, only an hour after Prime Minister George Rallis, the election's big loser, resigned.

Determined to waste no time to start on what he has termed "the road toward the great change" in Greek society and politics, Papandreou, once a U.S. citizen and a chairman of the economics department of the University of California at Berkeley, planned to be sworn in as premier Wednesday morning after announcing his Cabinet choices.

News services reported early Wednesday that Papandreou did announce a new Cabinet led by staunch members of the Pasok Central Committee.

The new prime minister appointed actress Melina Mercouri as minister of culture and sciences.

The foreign minister will be Ioannis Haralambopoulos, 62, a retired Army officer and engineer. Haralambopoulos has sat in Parliament for Pasok since it first participated in elections shortly after its foundation in 1974. He also has been a representative to the European Parliament since Greece joined the European Community in January.

Apostolos Lazaris, 60, will be minister of economic coordination. He is professor of economic analysis at the Higher Industrial School of Piraeus, and has been described as Papandreou's close collaborator and top planning man. He is expected to direct the sweeping economic changes Papandreou promised in his campaign platform.

Most of the new Cabinet members shared Papandreou's bitter opposition, from jail and later exile, to the colonels whose dictatorship began in 1967 and collapsed in 1974.

Papandreou, son of the late prime minister George Papandreou, has vowed to transform conservative Greek society by nationalizing banks and such key industries as pharmaceuticals and cement, reforming education health care and social security and decentralizing the country's byzantine bureaucracy.

He also has taken a strongly nationalistic line on foreign policy, questioning the terms of Greece's recent entry into the European Common Market, its long NATO membership and the presence of four U.S. military bases on Greek soil.