NEW YORK, OCT. 19 -- Testifying at the trial of four white teen-agers accused of assaulting him and two other blacks, Timothy Grimes bolted from the witness stand today shouting to a defense attorney, "You can't charge me, {expletive}; I'm finished with this {expletive}!"

Grimes, 18, was the first of three key prosecution witnesses to take the stand. Two white youths are charged with murder, and two with manslaughter, in the death of Michael Griffith, a 23-year-old black man who was killed by a car on a highway as he and two companions ran from a bat-wielding mob.

Grimes' outburst came after an hour of cross-examination by Stephen Murphy, one of four defense attorneys. Murphy's questions, most of which were ruled impermissible by New York State Supreme Court Judge Thomas A. Demakos, dwelled on Grimes' previous arrests for robbery and assault, and his use of marijuana and "crack."

Murphy, who promised last week to question Grimes "until I break him," represents Michael Pirone, 17. Pirone and Jason Ladone, 16, are charged with manslaughter, assault and riot. Jon Lester, 18, and Scott Kern, 18, are charged with murder, assault, riot and conspiracy.

Last week, Grimes told the jury that 10 whites with sticks had approached the three yelling, "Get the nigger." He said he brandished a knife in self-defense.

At the outset of the trial, defense attorneys said they would try to portray the blacks as unsavory and the aggressors.

Grimes began testifying Thursday and lost his temper twice during testimony today. As the judge called a brief morning recess, Grimes suddenly shouted, "Finished, okay? Finished, everyone!" and gave a one-armed Nazi-like salute.

Resuming his testimony later, he was questioned about the night of the attack and shouted repeatedly, "Finished!" Four court officers tried to subdue Grimes while Demakos ordered him back to his seat. When Grimes refused, the officers ushered him out of the courtroom.

Edward Boyar, an attorney with the office of Special Prosecutor Charles Hynes, said Grimes, who took the stand again in the afternoon, was "taking a beating, but that's okay. We have other witnesses who will corroborate what he said happened that night."

Cedric Sandiford, 36, the other survivor who was beaten by the white mob, will take the stand later this week.

The main prosecution witness is expected to be Robert Riley, one of the Howard Beach youths who agreed to cooperate with the special prosecutor.

Since opening arguments on Oct. 7, the most controversial testimony before today was the playback of a 911 emergency phone call in which a witness described white teen-agers beating a black man with a crowbar.