The Argentine government acknowledged today that it had detained Associated Press reporter Oscar Serrat for "interrogation" yesterday.

The government had denied any knowledge of Serrat's whereabouts when he disappeared yesterday morning on his way to work. The journalist, 45, reappeared at his home early this morning after being held for 17 hours.

According to AP spokesmen here, Serrat, who is an Argentine citizen spent yesterday, blindfolded and handcuffed, in what he believed was a military installation outside the city.

Serrat reportedly was questioned about the internal operation of the AP bureau, its receipt of guerrilla communiques - which most foreign news agencies here receive through the mail - and he knowledge of Rodolfo Walsh, a leftist Argentine journalist who has not been seen since he disappeared last spring.

Serrat's disappearance brought strong protests from the U.S. embassy and, following requests for assistance in locating him from AP headquarters in New York, an expression of concern from high levels of the U.S. State Department.

Serrat, who has worked for the AP for 14 years, was the first journalist working for a foreign agency here known to have disappeared. At least 35 local reporters have vanished over the past four years, and at least 25 have been reported killed in political violence under the previous civilian and current military government.

Some observers speculated that Serrat's abduction was related to yesterday's arrival of Raul Castro, the newly appointed U.S. ambassador to Argentina. Certain elements within the government are believed to oppose President Gen. Jorge Rafael Videla and, according to today's English-language Buenos Aires herald, "It is not too ridiculous to wonder whether the kidnaping of a top journalist working for a U.S. news agency was arranged to coincide with the arrival of a new U.S. ambassador in order to embarrass the government."

Others speculated that his kidnaping was an isolated, unofficial act ordered by an individual police official.

Last night, in a separate case, the military junta announced that Jacobo Timerman, imprisoned ex-publisher of the leading newspaper here, LaOpinion, has been relieved of his civil rights and control over all his assets and can be held in jail indefinitely.

Timerman was arrested last spring for alleged involvement in a financial scandal in which a banker David Graiver, killed last year in a Mexican plane crash, reportedly handled large sums of money for guerrillas.

Although a military tribunal recently cleared Timerman of involvement in subversive activities, he has continued to be held in an army prison for what a government spokesman today described as "economic" crimes.

A source close to Gen. Videla recently said that Timerman was being held for printing "bad ideas," an apparent reference to the newspaper's converage of liberal, and occasionally leftist, activites.

Timerman's imprisonment has been widely criticized by human rights groups and Jewish organizations. President Carter's human rights coordinator, Patricia Derian, brought up the Timerman case during a visit here last August.