A federal judge ruled yesterday that an internal 1980 Fairfax County police report given to the press by a county employe was a public document under Virginia's Freedom of Information Act.

The decision by District Judge James C. Cacheris means a six-member jury hearing a complaint by the employe, Robert E. Jurgenson, that he was improperly demoted and transferred for his actions, is expected to be instructed to disregard county claims the leaked document was confidential.

The ruling was a key victory for Jurgenson, whose lawyers have sought to show that department policy regarding the confidentiality of such reports was vague and at odds with the Virginia FOI law. Last night Victor M. Glasberg, Jurgenson's chief attorney, called the judge's decision "spectacular."

The report, completed in August 1980, detailed manpower, equipment and training shortcomings in the police department's Emergency Communications Center, where Jurgenson was a shift supervisor.

Jurgenson testified this week that he gave the 63-page internal audit to a Washington Post reporter in May of last year out of frustration at a lack of improvements after the critical report was circulated within the department.

Cacheris, citing a 1980 Georgia case later decided by the 5th U.S. Circuit Court of Appeals, said police regulations "can't supersede" a state's freedom of information law when the two disagree. "There is no exception in the Virginia law to protect this document in the public domain," the judge said.

The jury, expected to begin deliberations this morning, will decide whether senior Fairfax police officials acted properly in granting what they called a "voluntary demotion" and transfer to Jurgenson after he acknowledged leaking the report.

Jurgenson, who testified he agreed to the action under duress, later asked county Police Chief Carroll D. Buracker to rescind the decision. Both sides said this week that the chief refused.

Deputy Chief Thaddeus L. Hartman testified yesterday that Jurgenson, who is asking $250,000 in damages, has so far incurred an estimated $2,167 in lost salary as a result of the demotion.

Hartman, whose office prepared the report in mid-1980, testified earlier that none of the 15 or 16 such internal inquiries conducted by his staff over the past three years ever had been made public.

Evidence presented by Jurgenson suggested the police department denied the contents of the report even to county budget officials who were considering a police request for 12 more employes to strengthen the center's staff. Glasberg argued the report was suppressed because it embarrassed senior police officials.

Paul Puff, former commander of the communications facility, testified yesterday he had made a copy of the report available to the center's supervisors with a warning that it was confidential and was not to be removed from the room. Under cross- examination by Glasberg, however, Puff said he could not recall whether he issued the warning orally or in written form, whether the supervisors were warned individually or in groups or who the supervisors were.

"I knew he Jurgenson would like to have my position and call all the shots," Puff said. Jurgenson was an assistant supervisor at the time.