A cab driver who said he witnessed the slaying of Philadelphia police officer Daniel Faulkner last December positively identified Mumia Abu-Jamal yesterday as the man he saw standing over the officer as several gunshots rang out.

"I know who shot the cop, and I ain't going to forget it," testified Robert Chobert, 23, shortly after identifying Abu-Jamal, in the courtroom, as the man who shot the police officer.

Chobert, considered a star witness by the prosecution, admitted on the stand yesterday that he did not actually see a gun in Abu-Jamal's hand or see the firing of the bullets. He did say, however, that he saw Abu-Jamal's hand jerk back several times and that he heard shots after each jerk.

Chobert, who drives a cab for the Crescent Cab company, testified that he had just dropped off a fare at 13th and Locust Streets on the night of Dec. 9 when he heard a shot and saw a police officer fall down. He said he got out of his cab and walked toward the sound of the shots, about two car lengths away. At that time, he said he had seen Jamal standing over the officer before the suspect turned and ran about 10 feet away from the downed policeman and collapse.

Although Chobert was steadfast in his identification of Abu-Jamal, a former radio news reporter, he admitted under intense cross-examination by defense attorney Anthony Jackson that he had been mistaken in some details when he initially spoke to police. At that time, he described the assailant as being " heavy- set" and about 6 feet tall. Abu-Jamal is about 5-feet-9 and weighs 170 pounds. He also said initially that Abu-Jamal had run about 30 feet before collapsing, but today said he had run only 10 feet.

In addition, he told Jackson that he had seen the shooting take place between two cars that were different from the two cars described by other witnesses.

The dramatic testimony ended a long day at the Abu-Jamal trial, which did not conclude until almost 6 p.m.

In his opening statement at the trial, prosecutor Joseph McGill said he would prove that Abu-Jamal " viciously and intentionally" shot Faulkner between the eyes at point- blank range while the officer was lying on the ground.

The fatal shot, McGill told the jury, was from only 12 inches away and " literally blew away his brains. "

Soon after the shooting - which McGill said was witnessed by at least two people who will testify - Abu- Jamal was heard boasting of his actions, McGill said, even telling Faulkner's police partner " I shot the m-f- and hope he dies. "

"This is a case of extreme arrogance and defiance, of a strange boastfulness of what he did," McGill told the jury. " He viciously and intentionally took the life of Philadelphia police officer Daniel Faulkner. "

Faulkner was shot Dec. 9 at about 3:45 a.m. Abu-Jamal was arrested at the scene.

Although Abu-Jamal was evicted from the courtroom on Friday for what Judge Alfred Sabo called his " unceasing disruptiveness," he was back in court yesterday and was quiet throughout the day. When asked by Sabo at the beginning of the day whether he would behave, Abu- Jamal replied, " Sure, judge. "

The businesslike atmosphere yesterday was in sharp contrast to Friday's session, in which Abu-Jamal disrupted McGill's several attempts to address the jury. After Sabo evicted him, a brawl ensued in the courtroom between two of Abu-Jamal's brothers and the sheriffs. The two brothers were found in civil contempt and were summarily sentenced to 60 days in prison.

In another apparent change in strategy, Abu-Jamal also allowed his attorney, Anthony Jackson, to put on an active defense for him yesterday. Jackson had said Friday that he would mount no defense for Abu- Jamal because his client had been ordered not to. But yesterday Jackson told Sabo that he would defend his client in the traditional manner " unless and until Jamal tells me not to.

Jackson chose not to make an opening statement to the jury yesterday, saying that he instead would wait to do so until the end of the prosecution case.

McGill began his case by bringing on Maureen Faulkner, Daniel Faulkner's widow, to identify his police hat and several other items. She was composed on the stand. But for much of the day, the jury heard testimony from the police officers who responded to Faulkner's initial call for a police wagon at 13th and Locust for help with a " stopped car. "

Officer Robert Shoemaker, who was the first to arrive at the scene, said that he had seen Abu-Jamal sitting on the curb, with his hand on his chest. Abu-Jamal, it was later learned, had been shot.

Shoemaker said that he ordered Abu-Jamal to freeze, but that instead he began to move his hand toward what Shoemaker said he soon realized was a revolver. At that point, Shoemaker said, he kicked Abu-Jamal in the face, and then kicked away the revolver.

Shoemaker and his partner, officer James Forbes, also told of finding Abu-

Jamal's brother, William Cook, just feet from the body of officer Faulkner. Both said that they had heard Cook say, " I ain't got nothing to do with this. "

Forbes further testified that he had brought two revolvers that he found at the murder scene to the police ballistics lab. He said that one of the guns - believed to be Faulkner's service revolver - had one spent cartridge while the other - believed to be Abu-Jamal's gun - had five spent cartridges in it.

In his opening statement, McGill had claimed that Abu-Jamal had fired " numerous" shots at Faulkner.

In his cross-examination of the police witnesses, Jackson asked numerous questions about whether the officers beat Abu-Jamal or his brother when they arrived. All the policemen said that they did not, but officer Daniel Soboloski did say that Abu-Jamal was unintentionally pushed into a pole as he and two other policemen tried to bring him to the police wagon.