A D.C. Superior Court grand jury indicted a D.C. police officer yesterday in the assault last April of a Silver Spring man who reported that he was beaten while being held at a police station on disorderly conduct charges.

The grand jury returned the indictment against Officer Tommy C. Mustgrove, 26, of 12603 Lampton La., Oxon Hill. It accuses him of simple assault on Michael Morris 22, of 404 Hensdale Ct. on April 4.

The grand jury's action yesterday marked the rare case of an indictment of a city police officer growing out of an allegation of brutality.

D.C. Police Chief Burtell M. Jefferson recently reported that citizen complaints, ranging from cases of verbal disputes to physical abuse, are leveled against his department at the rate of about one a day.

A preliminary report issued by the department several months ago revealed that of 1,863 complaints, 1,712 had been classified by the department as "unfounded."

The report also showed that 151 compllaints were found justified by police officials, and indicated that 133 officers were "counseled," five received letters of reprimand and one officer resigned.

Morris, the complainant in the April incident, was arrested outside the Trailways bus station at 12th Street and New York Avenue NW, charged with disorderly conduct and detained in the 1st District substation cellblock in Southeast Washington.

Morris alleged that a desk sergeant first hit him on the hands while he was handcuffed and that later a second officer, Musgrove, came to his cell and hit him with a blackjack.

As a result of the alleged assault, Morris was treated at D.C. General Hospital. Three stitches were required to close a cut over his eye, sources said.

Morris did not file a complaint, however, until two other 1st District police officers -- Sgt. Earl K. Bell and Officer Benjamin Ash -- became aware of the incident and complained about it to their superiors more than a week later, according to sources.

James Owens, chief of the U.S. attorney's misdemeanor trial section, said his office launched an investigation after hearing complaints from the two veteran police officers.

According to police, their investigation showed that "force" was used against Morris after he "resisted" police attempts to fingerprint him.