NEW YORK, OCT. 22 -- In a thick, monotone Queens accent, Robert Riley described today how he and his friends chased a black man to the edge of the Belt Parkway in Howard Beach last December, testifying that he heard a loud "thud" and saw the man's body "go up a couple of feet in the air" when he was struck by a passing car.

Riley, 18, the prosecution's chief witness at his friends' trial, said that he and four other white teen-agers had pursued Michael Griffith to a roadside guard rail and that Griffith ran "straight onto the Belt Parkway" where he was hit.

Riley said the chase began several blocks earlier when he and his high school friends confronted three black men outside a pizzeria and ran after them, shouting, "Niggers, get the {expletive} out of the neighborhood."

One of his friends, Jon Lester, was wielding a metal bat when the chase stopped at the parkway, Riley said.

Riley, who is under 24-hour police guard, is being allowed to plead guilty to a lesser charge of assault, which carries a maximum seven-year prison term, in exchange for his cooperation. The son of a city corrections officer, Riley originally was charged with second-degree murder.

Riley's account was the first during the seven-week trial in Queens State Supreme Court to place the defendants -- Lester and Scott Kern, charged with second-degree murder, and Jason Ladone and Michael Pirone, accused of manslaughter -- in pursuit of Griffith at the parkway when he died. The special state prosecutor contends that the defendants caused the death of Griffith, 23, because he had no choice but to flee onto the parkway.

The contentious trial often sounds like a New York barroom argument as the four boisterous defense lawyers loudly object to nearly every line of questioning and Justice Thomas A. Demakos tries to quiet them. On Monday, caustic questioning by defense lawyer Stephen Murphy twice caused Timothy Grimes, 19, one of the black victims, to lose his temper and leave the witness stand muttering obscenities.

Tempers flared in the jammed courthouse lobby today as angry black spectators began pushing while dozens of Italian teen-agers from Howard Beach were led into seats reserved for the defense. "Let some black people in," several shouted.

Riley, a baby-faced witness with close-cropped hair, stood before blown-up photos of Cross Bay Boulevard as prosecutor Edward Boyar led him through the events of Dec. 20, 1986, which turned Howard Beach into a national symbol of racial strife.

Riley said he first went to a party where he drank "10 or 12 beers." At about 12:20 a.m., he said, Lester returned to the party and said "that there were some niggers on the boulevard -- let's go over and kill them."

Riley said he and several friends found the three black men near a local pizzeria. Two of the blacks displayed knives, he said, while Kern "had the bat in his hands. He was banging it on the floor . . . . They formed half a circle and the black guys formed the other half of the circle . . . .

"I grabbed the bat out of Scott's hand. I said, 'Give it to me; I can swing it harder than you could.' The black guys started running . . . . We chased after them."

Riley said he jumped in a car and picked up the chase on a street leading to the parkway. He said Lester demanded the bat and, after a scuffle, "he managed to get it."

Riley said that he and the four defendants chased Griffith "towards the parkway" and that Griffith was "10 to 15 feet" ahead of them. He said he saw Griffith clear the guard rail, run onto the parkway and jump the center divider. He "got hit on the first lane," Riley said.

Asked if he heard any sound, Riley said, "I could hear it from the end of the block . . . . It was like a thud."

Riley, who now works in a liquor store, acknowledged involvement in past incidents in which he squirted a rabbi with water from a fire extinguisher and used a red police light to pull drivers over.