NEW YORK, DEC. 21 -- Three white teen-agers were convicted of manslaughter tonight for chasing a black man to his death on a Queens highway in Howard Beach last December in one of the most racially charged cases in New York history. A fourth defendant was acquitted of all charges.

After a three-month trial and 12 tense days of deliberations, a state Supreme Court jury rejected second-degree murder charges against Jon Lester and Scott Kern, both 18, while finding Lester, Kern and Jason Ladone, 17, guilty of second-degree manslaughter in the death of Michael Griffith, 23.

Michael Pirone, 18, was acquitted of manslaughter and lesser charges.

The jury, which included one black, also found Lester, Kern and Ladone guilty of first-degree assault in the baseball-bat beating of another black, Cedric Sandiford, 37, that night. The jury rejected attempted-murder charges in that incident.

Relatives gasped and sobbed as the guilty verdicts were pronounced, and some blacks in the packed courtroom applauded. After the verdicts were read, a black man shouting "murderers!" was tackled by a half dozen security guards and dragged from the courtroom as were two shouting whites.

Outside the courthouse, several black activists said they welcomed the verdict and praised the prosecution's handling of the case.

Griffith was struck by a car on the Belt Parkway Dec. 20, 1986, after fleeing a gang of whites shouting "Niggers, get the {expletive} out of the neighborhood." That night's events turned Howard Beach into a national symbol of racial violence and set the stage for the emotion-charged trial that ended at 8:30 tonight.

The manslaughter and assault charges on which three of the defendants were convicted carry separate terms of five to 15 years in prison. The murder charge of which they were acquitted has the term of 25 years to life.

Justice Thomas A. Demakos released Kern and Ladone on bail and set their sentencing for Jan. 25. Lester, serving one to three years on an unrelated weapons charge, was ordered to return for sentencing Jan. 20.

Pirone told reporters after his acquittal that he was "happy and joyful" and that his lawyer had "proved there was no evidence against me." But, he said, "I feel bad for my friends . . . . there should have been four acquittals."

Defense lawyers vowed to appeal.

"Sometimes, justice takes more than one step," said Ronald Rubinstein, Ladone's lawyer. "We had to move these people {the jury} 360 degrees because of preconceived notions before the trial started. Unfortunately, after 12 days they succumbed to the hysteria the politicians and the media created."

Kern's lawyer, Gabriel Leone, said he was "stunned" by the verdict. He said that the judge had committed numerous errors and that "the evidence was insufficient, totally."

Special state prosecutor Charles J. Hynes said the verdict sent a message that "mindless intolerance must end." He said he had promised Sandiford months ago that, while "I could never bring Michael back, I would do everything I could to punish the killers who took Michael away."

Sandiford and Griffith's mother, Jean, 42, of Brooklyn, praised Hynes' efforts. "There is no word really to express the thanks I feel in my heart," Sandiford said.

Under state law, the manslaughter convictions hinged on the jury finding evidence to corroborate the testimony of star prosecution witness Robert Riley, 18, who was charged with second-degree murder in the attack but will plead guilty to assault under a plea-bargain agreement.

Outside the courthouse on Queens Boulevard, about 30 blacks -- some carrying a banner that said "Say no to racism from Howard Beach to Johannesburg" -- shouted at a beefed-up police contingent.

Earlier today, hundreds of demonstrators had blocked Brooklyn-bound traffic on the Brooklyn Bridge during evening rush hour and stopped key subway lines in defiance of a court order in what black leaders called a "day of outrage" to protest the Howard Beach incident and what they called racism in the city.

Authorities said more than 70 persons were arrested for blocking subway tracks. Protesters halted trains in three Brooklyn stations by pulling emergency-brake levers and holding doors open, preventing trains from leaving stations, officials said.

Defense lawyers unsuccessfully sought a mistrial today after the disclosure that a television news producer has been acting as an agent for the jury foreman in peddling her story to the highest bidder.

Justice Demakos refused to question foreman Nina F. Krauss, 28, a paralegal from Forest Hills, Queens, about the report in today's New York Daily News, defense attorney Steven Murphy said.

Mark Friedman, a field producer for WNYW-TV (Channel 5) who has been covering the trial, contacted the Daily News over the weekend and said Krauss has been keeping a diary of the deliberations and had authorized him to sell her story, the newspaper reported. Friedman claimed that the New York Post had offered $7,500 for Krauss' story and Newsday $10,000.

While it is not unusual in New York for jurors in highly publicized cases to sell their accounts to the news media, it is felt that such negotiations could influence deliberations improperly if they begin before a verdict is returned.

A WNYW spokesman said that "the station had no knowledge of Friedman's actions, which are a violation of station policy" and that Friedman has been suspended.

The Howard Beach assault, which Mayor Edward I. Koch called a "modern-day lynching," attracted intense publicity and led to appointment of Hynes as special state prosecutor. In his summation, Hynes said the defendants had unleashed a night of "fists, fear, terror and hatred" and chased Griffith into a "chasm of death."

The defense depicted the events as a turf fight in the predominantly white Queens neighborhood, not a racial incident, and called Griffith's death accidental.

Finding Lester and Kern guilty of second-degree murder, Demakos told the jury, required proof that they demonstrated "a depraved indifference to human life" and "recklessly engaged in conduct which created a grave risk of death" through "brutal," "callous" and "inhuman" actions.

The manslaughter charge required only a finding that the defendant "recklessly caused the death of another person" and ignored the risks involved, Demakos said.

The basic facts were uncontested. Three blacks -- Griffith, Sandiford and Timothy Grimes, 19 -- walked into a Howard Beach pizzeria after their car broke down nearby. They became involved in a confrontation with a group of white teen-agers, who began chasing them while shouting insults.

The defendants chased Griffith, a Brooklyn construction worker, toward the Belt Parkway, where he was struck by a car after he jumped the center divider. Then they beat Sandiford with a baseball bat and tree limbs, opening head wounds that required 66 stitches.

There were diverging accounts of how this occurred and who was responsible. Hynes portrayed the teens as vicious brutes who left a beer party to confront the blacks and caused Griffith's death by leaving him no avenue of escape.

When Sandiford pleaded, "I've got a son like you . . . Please don't kill me," Hynes said, "his response was to have a baseball bat smashed across his face."

Defense lawyers, however, described the blacks as troublemakers with criminal records, "three antagonistic men spoiling for a fight," as attorney Bryan Levinson put it.

They emphasized that Griffith had been a cocaine user, that Sandiford had been convicted on gun charges and that Grimes, subsequently charged with stabbing his girlfriend, had faced assault and burglary charges.

Hynes called this denigration of the blacks a "smoke screen," saying they did not need "a passport to get into Howard Beach."

Defense attorneys picked apart much of the testimony, spotlighting inconsistencies. Sandiford, for example, testified that his bat-wielding assailant had blue eyes but continued to insist that the brown-eyed Lester attacked him.

Grimes had told a grand jury that he was not carrying a weapon that night but acknowledged at the trial that he had brandished one after the whites confronted him at the pizzeria.

Under aggressive questioning by Steven Murphy, Pirone's lawyer, Grimes twice bolted from the witness stand, shouting, "Finished! I'm finished with this {expletive}!"

Using enlarged photographs of surrounding streets, defense lawyers also highlighted conflicting accounts about the point from which Griffith ran onto the parkway and where the white youths were standing at the time.

"Racism has nothing to do with this trial," Murphy said in his summation. "It was an accident brought about by stupidity."

Prosecutors called 61 witnesses and defense lawyers 13 during the three-month trial. None of the defendants took the stand.

After Hynes successfully challenged the defense practice of excluding blacks from the jury without reason, one black juror was seated, along with six whites, two Hispanics, two Asian-Americans and an Indian from Guyana.

Because of a family problem, the black was excused from the case hours before deliberations and was replaced by a black alternate.

Special correspondent Marianne Yen contributed to this report.