A corrections expert yesterday labeled conditions in Prince William County's jail as "deplorable and the overcrowding incredible . . . If they kept animals that way, the SPCA would swear out warrants all over the place."

John Case, testifying in a class-action suit brought by two ex-prisoners against the county's Board of Supervisors and Virginia prison officials, said he found the jail marked by violent assaults and its weaker prisoners living in tension and fear.

"The Prince William conditions are as bad as any jail I've ever been in," said Case, field director of the Pennsylvania Prison Society, a prison reform group, and the former director of corrections in Bucks County, Pa., near Philadelphia. "Maybe it's the worst I've been in. They're at the bottom."

The lawsuit was brought in U.S. District Court in Alexandria on behalf of an estimated 6,500 to 7,000 prisoners held in the Prince William jail between August 1980 and Jan. 22, 1982. It contends conditions at the jail violated constitutional guarantees of due process and protection against cruel and unusual punishment.

Yesterday's testimony before a six-member jury came on the eve of closure of the 32-year-old, two-story brick prison and the imminent opening of a new $5 million, 160-cell detention center, which was dedicated by county officials in Manassas Friday.

An attorney for the seven-member Prince William Board of Supervisors acknowledged in an opening statement yesterday that overcrowding exists in the current jail. But, said lawyer James A. Welch, "We specifically deny that the sheriff, jailers, guards or the board ever did anything to block the betterment of jail facilities.

"There was no intentional conduct of any kind to harm individual human beings."

The board has filed its own suit against Virginia corrections officials, claiming the county was a victim of the explosion in the state prison population and that Virginia officials failed to act to relieve the crisis.

Case said that in a Jan. 18 tour of the Prince William jail he found prisoners sleeping on mattresses on the floor, inadequate sanitation and plumbing, poor medical facilities and lack of guard supervision.

There were no planned activities, no educational programs, no volunteer programs, "and no room for any," Case said. The jail, built to hold 30 prisoners, held an average of 85 in 1980, 98 last year and 94 in January, according to county figures presented by the prisoners' lawyer.

On cross-examination, Case acknowledged he did not interview personnel at Prince William Hospital, where prisoners are taken for medical care, did not review individual health records and had no direct evidence of alleged telephone privileges being denied or proof that limited visiting hours fueled tension and anger in the jail.