A 20-year-old mental patient seized a policeman's service revolver and fatally shot himself in a room at Fairfax Hospital yesterday as his mother watched, police reported.

The shooting occured, police said, after Phillip Hall became violent in his room, tried to jump out a window and began attacking his mother.

Officer Ray M. Clements of the county police force, who had been assigned to guard Hall, rushed into the room and struggled with the patient, but Hall ripped Clements' revolver from its holster and shot himself in the chest, according to police.

About an hour before, a magistrate had ordered Hall committed to a state mental hospital in Staunton, Va. Hall was temporarily placed in a regular hospital room under guard because the Fairfax Hospital's psychiatric ward was full.

The shooting prompted County Supervisor Alan H. Magazine (D-Mason) to question why a policeman guarding a mental patient would be carrying a gun.

The County police chief, Col. Richard A. King, said he plans to evaluate the policy of having armed police on such guard duty after he receives the results of a police investigation of the shooting.

Fairfax Hospital spokeswoman Peggy Pond said Hall was one of about 250 mental patients who must be kept in private rooms under armed guard each year because the hospital's 32-bed psychiatric ward is almost always full. Such patients are kept for about a day and a half, pending commitment to a state mental hospital, Pond said.

Hall, who was ordered committed to Western State Hospital in Staunton by Special Magistrate Michael Valentine yesterday morning, had been taken to Fairfax Hospital late Sunday by a friend, Pond said.

In commitment papers filed yesterday, Dr. Claude E. Cooper, a physician employed by Fairfax in commitment proceedings, wrote that Hall "is delusioned, believing he is God or the Son of God. He does things God tells him to do. He [recently] worked to restore an old house then tore it up."

His speech is confused and doesn't make sense. Hospitalization for observation and/or diagnosis and treatment is recommended," Copper wrote.

Fairfax police and the hospital had little information yesterday about Hall, who used to work as a carpenter and who police said had no fixed address.

Police said Officer Clements, 27, rushed into the hospital room at 9:30 a.m. after he heard Hall's mother, Wilma G. Hall, scream for help. Mrs. Hall, who alleged in commitment papers that her son was "unable to care for self," could not be reached yesterday following the shooting.

Pond said that Fairfax Hospital only agrees to accept "dangerous" patients when the county police or sheriff's office provide armed guards.

"Of course, it is not an ideal situation," Pond said, "but it is the best we have at the present." The armed guard usually stands outside the mental patient's private room, Pond said.

Police Chief King said "it is indeed unfortunate that there was not sufficient space" in the psychiatric ward. He added that he would prefer that his officers did not have to guard the mental patients, but his department has no choice.

County police occasionally enter the hospital's psychiatric ward, but the hospital forbids them to carry guns when they do. When mental patients are placed on an "open ward," King said, his police wear the same equipment they wear on the street.

"Certainly the same kinds of things [an enraged person grabbing for a policeman's gun] could occur in the conduct of an arrest," King said. County police policy requires policemen to relinquish their revolvers only when they are fingerprinting and photographing suspects at police headquarters.

Objecting to the policy of having armed police near mental patients, Supervisor Magazine said, "It would seem to me that there are weapons available to guards in these situations that are not lethal."

Magazine said "something is wrong" with the county's supervision of mental patients if armed police come in contact with potentiall uncontrollable patients on a regular basis.

Commitment papers filed in Fairfax County General District Court said Hall had a history of psychiatric treatment at the Woodburn Center for Community Mental Health.

The papers also said Hall was a user of marijuana, cocaine and "KW" an abbreviation for Killer Weed-marijuana treated with the animal tranquilizer PCP.