More than 65 undercover and uniformed Prince George's County police officers joined almost 20,000 rock fans at the Capital Centre last night and made drug arrests while the rest of the audience listened to the rock group Heart.

Twenty-one persons -- 13 adults and eight juveniles -- were arrested on drug-related charges in the second "raid" in four months at a Capital Centre rock concert. It was also the sixth in a series of crackdowns begun several years ago in an effort to undermine the Largo arena's reputation as a safety zone for the drug subculture.

Police infiltrated the crowd last night and quietly escorted suspects from the sold-out, mostly teen-age audience to waiting paddy wagons outside. Police said they were most interested in arresting drug pushers.

Large quantities of drugs including marijuana, hashish, PCB, LSD, and amphetamines were seized, along with thousands of dollars in cash, according to police.

The county police drug strike force was assisted by mounted police officers from the Maryland-National Capital Park and Planning Commission force who were used to disperse small groups of people who gathered around the police and their suspects outside the arena.

Most spectators appeared unaware that arrests were being made.