As U.S. attorney, Joseph E. diGenova usually prosecutes criminals. Yesterday he tried to prevent new ones from developing, when he visited a Northwest Washington junior high school to launch a program to steer youths away from illegal drugs.

DiGenova told 800 teen-agers in Shaw Junior High School's auditorium that "the community loves you and cares about you," but warned that if they didn't "say no to drugs," they could be arrested and jailed.

"There are so many young people involved in drugs, our office is very busy [prosecuting cases]. We don't want to be that busy," said diGenova, who told the students that the Black Assistant U.S. Attorneys Association had created the new antidrug program and enlisted his help.

The program, which supplements antidrug programs run by the school system, comes when the use of phencyclidine, or PCP, a cheap, potent liquid usually mixed with marijuana and smoked as a cigarette, is increasing at an alarming rate among Washington area teen-agers and young adults.

Arrests for possession or sale of PCP have increased in the District from 310 four years ago to 3,030 last year.

Students at inner-city schools such as Shaw, at 10th Street and Rhode Island Avenue NW, are "extremely vulnerable" to drugs, said D.C. School Superintendent Floretta D. McKenzie, who spoke to the students and praised the program.

"When people in the community offer to help out before youths get in trouble," she said, "that's always positive."

Under the program, prosecutors from the U.S. attorney's office, plus representatives of D.C. General Hospital and Rap Inc., a drug rehabilitation center, will visit two schools each month to explain to students why they should not use drugs such as marijuana, PCP or cocaine, and tell them what happens to those who say "yes" instead of "no" to drugs.

The Rap Creative Play Workshop, a group of former drug addicts who perform skits about the city's drug subculture, and D.C. police Detective Johnny St. Valentine Brown Jr., better known by his nickname "Jehru," an expert on narcotics, received the most enthusiastic student response.

"A lot of kids use drugs because they think it makes them hip," Brown said. "But it's hip to say no."

He added, "I've seen young people under the influence of PCP actually tear their eyes out of their sockets."

Brown found the audience knowledgeable about slang terms used for illegal drugs. "PCP has many different names," he told the students. "You know any?"

"Lovely, Love Boat, Angel Dust," the audience shouted.

"Butt Naked," several added.

"Yes, it will definitely make you undress," Brown said. "I've seen people tear their clothes off and run down the street under the influence of that drug. It is most dangerous. It will literally fry your brain."

Fred Douglas, president of the Black Assistant U.S. Attorneys Association, said that the program was conceived to give students a realistic view of the perils and punishments associated with illegal drug use.

"Not only was it [the program] informative," Douglas said later, "but it seemed to hold the attention of the students."