The American Federation of Government Employees, which represents about 700,000 federal workers, filed suit against the government yesterday charging that mandatory secrecy pledges violate employes' constitutional rights.

The lawsuit, filed in U.S. District Court here on behalf of the union members and three Defense Department workers named in the suit, asks the court to declare the pledges illegal and to rescind the secrecy agreements signed by more than 2 million federal employes.

The union argues in the lawsuit that the restrictions interfere with employes' freedom of speech and that they will inhibit employes who want to blow the whistle on fraud, waste and abuse in government.

At issue are two types of secrecy pledges.

The more common pledge, which applies to 3 1/2 million to 4 million government employes and contractors with access to classified information, requires those workers to promise not to disclose classified or "classifiable" information. That pledge, known as Standard Form 189, is overseen by the Information Security Oversight Office, a part of the General Services Administration.

The second pledge, which applies only to employes with the highest-level clearances -- those covering Sensitive Compartmented Information (SCI) -- requires such workers to sign a lifetime pledge stating that they will obtain approval from government censors for any book, speech or publication, including fictionalized accounts, dealing with classified material. That pledge, known as Form 4193, applies to about 150,000 current workers with SCI clearances and is overseen by the Central Intelligence Agency.

Employes who refuse to sign either pledge can lose their security clearances and ultimately their jobs.

The SCI pledge was first instituted in 1981 under the Reagan administration at the request of the CIA. In 1983, President Reagan issued a national security directive expanding the use of polygraph examinations in investigations of leaks, requiring the secrecy agreement for employes with SCI clearances and instituting a second pledge, Form 189, for employes with lower-level clearances.

After a storm of criticism over the polygraph provisions and the SCI pledge setting up the pre-publication requirement, Reagan rescinded in early 1984 those parts of his directive. Government agencies have continued to require holders of SCI clearances to sign the 1981 version of the pledge, which is almost identical.

One plaintiff named in yesterday's lawsuit is Louis Brase, a civilian Air Force employe at Goodfellow Air Force Base in Texas who until last month held an SCI clearance. He worked as a training manager instructing Air Force personnel in using cryptographic equipment.

Brase, who has worked for the Air Force for 35 years, signed his SCI secrecy pledge last year, but refused in mid-August to sign Form 189 because of questions about the word "classifiable."

"My interpretation was that if at any time I were to write or speak something that displeased the government, they would have the opportunity to declare that information classified and charge me with violating the agreement," he said.

Named as plaintiffs with Brase are Jim Stinchcomb, a government missile handler, and James H. Douglas, a Navy firefighter.

On Aug. 17, the 150,000-member National Federation of Federal Employees filed a lawsuit asking that Form 189 be declared unconstitutional, largely on the basis of questions about the interpretation of the term "classifiable."

Steven Garfinkel, who runs the GSA security office, has pointed out that the definition of classifiable was clarified -- in the Aug. 11 Federal Register -- to mean information that was classified but had not been so designated.

But as a result of the August lawsuit, he sent letters to government agencies last week instructing them not to withdraw the security clearances of employes who refuse to sign Form 189.

In Brase's case, all of his clearances were suspended at first, and he found himself assigned to work as a typist. Since Garfinkel's letter went out, his lower-level clearances have been reinstated, and he is working as an educational counselor, a job three pay grades below his earlier one. Meanwhile, his SCI clearance -- for which he signed the pledge last year -- has been revoked.

The Air Force did not respond to requests for comment yesterday on the Brase case.