Rallying to the cause of a recently ousted clerical worker, civil rights and gay activists have launched a public assault on the FBI's traditional and "absolute" policy of firing any homosexual detected within its ranks.

Unknown numbers of homosexual FBI employes-often young, naive recruits fresh out of high school-have resigned quietly over the years in the face of pressure from the agency in order to avoid embarrassment, the activists claim.

But last month a 22-year-old fingerprint clerk, John Calzada, resisted those pressures and refused to resign. The FBI fired him May 1, stating as grounds for the action "your homosexuality."

With financial backing from the American Civil Liberties Union (ACLU) and the expressed support of at least two congressmen, Calzada has decided to appeal his dismissal to FBI director William Webster and take the case to court if necessary.

An FBI spokesman declined comment yesterday on Calzada's specific case but said FBI policy on homosexuality "is that such conduct is considered as an element of possible compromise of the employe and may be a factor in dismissing an employe considered unsuited for continued FBI employment."

Beyond the question of homosexuality, the case could also indirectly affect the FBI's strict rules of personal conduct for all employes which cover hair length and dress, and prohibit "immoral or disgraceful conduct, or other conduct prejudicial to the government."

The FBI has virtually total discretion in whom it fires and why. Congress has excempted it from civil service procedures that guarantee most other government employes at least a hearing if they are dismissed.

Franklin E. Kameny, a member of the D.C. Commission on Human Rights and a prominent gay activist, is representing Calzada is his administrative appeal.

Though not a lawyer, Kameny was a prime mover in the highly publicized case of Air Force Tech. Sgt. Leonard p. Matlovich, which challenged military policies toward homosexuals.

The Gay Activist Alliance, with support from Americans for Democratic Action, the D.C. Democratic State Committee, the mayor's office and other groups, has planned a demonstration against "FBI Homophobia" today, 4 to 6 p.m., at the J. Edgar Hoover Building on Pennsylvania Avenue.

Calzada said he decided to fight, rather than resign quietly, because, "I feel strongly about the issues. The FBI doesn't have any business probing into my private life. . . Times have changed."

He had not been a "flaunting" homosexual, he said, but last March mentioned to friends on the night shift that he and his lover (not an FBI employe) were planning a homosexual wedding.

Word eventually reached FBI officials. They called Calzada in and when he confirmed the rumors, he said they asked him to resign.

During meetings with officials at the FBI during March and April, Calzada said he was pressed for the names of other homosexuals at the agency. "Then when I refused, they started naming people, asking, 'Is he a homosexual?'"

The case of Donald Ashton, 24, is more typical of the fate of homosexuals at the FBI, according to Kameny and others. Ashton's case, also with ACLU backing, is now awaiting a decision by the Circuit Court of Appeals here.

Just out of high school in Uniontown, Pa., Ashton was recruited into the FBI as a mail sorter. In 1974, he met a Navy enlisted man at a night spot called Pier Nine. The two developed a friendship that culminated in what court records called a "homosexual encounter."

Later, during an investigation of the Navy man, Navy investigators found out and passed on to FBI officials the news that the FBI had on its payroll a "mail clerk named Don who was a homosexual."

The FBI set out to discover which of its employes fit this description, pulling the files of 55 employes named Don, according to court records.

They finally zeroed in on Ashton and summoned him for an interview in which they asked him detailed questions about the nature of the sexual encounter, records indicate. After a 90-minute interrogation Ashton handed the agents a handwritten resignation, he said.

The FBI maintains that the resignation was voluntary, but Ashton said the agency threatened to give him a bad job recommendation and embarrass him if he did not resign.

A U.S. District Court judge found that even if the resignation had been coerced, the FBI had the right to fire Ashton without a hearing, for reasons not related to job performance.

In the wake of the Calzada case, Rep. Don Edwards (D-Calif.), chairman of the House Judiciary subcommittee on civil and constitutional rights, and Rep. Ted Weiss (D-N.Y.) have both criticized the FBI policy and asked FBI officials to explain it to them.

"On its face, its discriminatory," Edwards said. "I think it's very inappropriate."

ACLU official Ralph Temple called the FBI policy a matter of "prejudice, a kind of visceral prejudice."

The civil service system, for purposes of security clearances and foreign relations, has established that "a person's private sexual preferences are not a grounds for disqualification from government service," Temple said. CAPTION: Picture, Former FBI fingerprint clerk John Calzada stands in front of the J. Edgar Hoover Building. By James M. Thresher-The Washington Post