Argentina's military government, which has been trying to change its image following extensive U.S. and international criticism of its alleged humanrights violations, has admitted that it is holding missing teachers' union leader Alfredo Bravo in detention.

The announcement Tuesday night came 13 days after Bravo, a prominent human-rights leader in the country, disappeared and hours after the Permanent Assembly for Human Rights made public a letter to President Jorge Videla demanding information on Bravo's whereabouts.

The letter from the assembly, of which Bravo is a co-director, was signed by various religious, political and intellectual leaders. It was the first time such a broad spectrum of prominent citizens has joined in a human-rights protest here.

The letter said Bravo was kidnapped at a local school Sept. 8 by men claiming to be federal police officers. Government officials subsequently denied knowing anything about his disappearance, which occured on the eve of a meeting between Videla and President Carter in Washington, where Latin American leaders were gathered for the signing of the Panama Canal treaties.

The strongly worded letter, dated Sept. 15, called on Videla to explain how "this new and reprehensible kidnaping could have occured with such manifest impunity" and noted that "the moment has arrived" to solve such case as a prerequisite to "whatever future development" is expected in Argentina.

Signers of the letter included former President Artuno Illia, Radical Party leader Richardo Balbin, Methodist and Catholic bishops, former goverment officials, and a number of prominent Argentine attorneys and intellectuals.

An Interior Ministry release said that Bravo is being detained in La Plats, southeast of Buenos Aires, but did not explain on what charges.

It said only that Bravo is held under "executive power" a form of detention used against, suspected subversives here that puts these uncharged prisioners at the disposal of the president without access to legal process.

Government sources estimate that at least 4,000 Argentines are now held under executive power. In the vast majority these cases, the names are not released and the prisoner's families are left to declare them missing, with no government response.

The Permanent Assembly for Human Rights, a group of well-known attorneys and intellectuals, is one of several local groups that have been active in human-rights protests here.

Early this year, U.S. relations with Argentina deteriorated after Carter recommended that military assistance be cut to a minimal $15 million because of human-rights concerns. In June Congress went a step further by voting to cut Argentina out of its 1977-78 foreign aid bill.

Argentina has since played cordial host to a series of official American visitors, and Carter recently praised a governmetn announcement that it would release more than 300 political prisioners.