ATHENS, Greece (AP) _ Constantine Karamanlis, whose resignation as president has created a climate of political uncertainty, returned to private life today, ending a 50- year career in politics.

The conservative politician, who supported close ties with the United States and NATO, announced his resignation Sunday after he was abruptly rejected by the ruling Socialists as their candidate in next week's presidential balloting.

A small crowd of admirers standing outside the presidential mansion in downtown Athens in drizzling rain applauded and shouted, ''Long live Karamanlis,'' as his limousine drove off to his home in Politeia, a northern suburb.

Sources close to Karamanlis said he was ''expected to concentrate now on his archives and the things he never had time for while in office.'' They asked not to be identified.

But Greek conservatives don't believe that Karamanlis will stay out of politics entirely, although they say he is unlikely to run for president again.

''He won't be allowed to remain uninvolved, even if he wants to, because people will want his advice and opinion. And after all, politics has always been Karamanlis' consuming interest,'' said Helen Vlachou, publisher of the conservative daily Kathimerini.

Karamanlis, 78, was first elected to Parliament in 1935, and had dominated the Greek political scene since he restored democracy in 1974 as premier after a military dictatorship collapsed.

He became president in 1980 and had been expected to become a consensus candidate for a second five-year term.

But in a last-minute switch, the ruling Panhellenic Socialist Movement's 140-member central committee chose Christos Sartzetakis, a 56-year-old Supreme Court judge, as their candidate, and decided to amend the constitution Karamanlis masterminded in 1975.

Analysts say the decision to replace Karamanlis represents a victory for Marxist radicals over moderates in the party, and a commitment to more sweeping social and political reform.

The revisions, submitted to Parliament on Monday, would make the president a figurehead and transfer his executive powers to the ruling party.

Karamanlis' departure has raised fears that the Socialists, who favor closer relations with the East bloc and non-aligned countries, will take Greece sharply to the left.

''The threat of Karamanlis' using his presidential powers combined with the weight of his personal counsel helped restrain the Socialists. Now the safety net has gone,'' said a Western diplomat who spoke on condition he not be identified.

Greece's Moscow-line Communist party, known by its Greek initials KKE, has pledged to back Sartzetakis for president in Sunday's parliamentary balloting.

That makes his election a near certainty and raises the possibility of future cooperation between the Communists and Premier Andreas Papandreou's Socialist party.

The Socialists have 165 seats in the 300-member Parliament and the Communists have 12.There are 11 independents.

Sartzetakis needs 180 votes for election in the third and final ballot. A two-thirds majority is needed on the first and second ballots.

The conservative opposition New Democracy party, which controls 112 seats, says it will cast blank ballots.

Sartzetakis, from the northern city of Salonica, is widely respected. As a young magistrate, he investigated a 1963 political killing in which left-wing deputy Gregory Lambrakis died in a road accident engineered by right-wing extremists.

The case inspired the film, ''Z'', directed by Costa-Gavras, in 1968.

During Greece's seven-year military dictatorship, Sartzetakis was tortured and jailed for one year without being charged.