With four bullets still in his body, a survivor of the 50-shot police barrage that killed an unarmed groom-to-be took the witness stand yesterday and belittled the undercover officer he accused of starting it all.

“This dude is shooting like he’s crazy, like he’s out of his mind,” Joseph Guzman testified at a trial of the undercover officers in Queens.

Mr. Guzman, 32, was referring to Detective Gescard Isnora, one of three officers charged in the slaying of Sean Bell outside a Queens topless bar on November 25, 2006 — the morning of Bell’s wedding day.

The key prosecution witness insisted that, contrary to police accounts, Detective Isnora “appeared out of nowhere” with a silver gun drawn, and never identified himself as a police officer.

Mr. Guzman recalled locking eyes with Detective Isnora, whom he repeatedly referred to as “that kid” or “dude.” Around the same time, he said, an unmarked police van collided with a car driven by Bell as shots rang out.

“That’s all there was — gunfire,” he said. “There wasn’t nothing else.”

Mr. Guzman, seated in the front passenger seat, was hit 16 times from the neck down; four bullets were never removed. At one point in his testimony, the witness paused to unbutton his shirt to display a circular scar on his right shoulder.

When the shooting stopped, Mr. Guzman said he spoke to Bell using the 23-year-old’s nickname, “S,” saying, “I love you.” He said Bell whispered “I love you” back. Then, he said, his longtime friend “stopped moving.”

Prosecutors have portrayed Detective Isnora and two other detectives, Michael Oliver and Marc Cooper, as reckless cowboys who were poorly prepared for an undercover operation targeting prostitution at the strip club where Bell was hosting a bachelor party.

Detectives Isnora and Oliver have pleaded not guilty to manslaughter. Detective Cooper has pleaded not guilty to reckless endangerment.

The detectives, wearing street clothes and trying to blend in with club patrons, say they became alarmed after witnessing an argument outside the club between a belligerent Bell and the driver of an SUV who appeared to be armed, then overheard Mr. Guzman say to someone, “Yo, go get my gun.” Detective Isnora, who tailed Bell, Mr. Guzman, and another buddy, Trent Benefield, on foot, confronted the men as they headed for their car, suspecting they might be retrieving a weapon.

He told a grand jury he identified himself as a police officer and was the first to open fire after Mr. Guzman made a sudden move and Bell bumped him with his car while frantically trying to pull away.

Yesterday, Mr. Guzman said he too believed the SUV driver was armed because the man put his hand in his right coat pocket and told Bell’s group, “I don’t fight no more.”

But Mr. Guzman also said he played the role of peacemaker outside the club, telling the SUV driver no one wanted trouble.

He also denied saying anything about having a gun because he didn’t have one that night.

“Where I’m from, that’s not a good bluff,” he said, raising his voice in one of several heated exchanges with the defense attorney, Anthony Ricco. “I don’t know where you come from.”

He later mocked Mr. Ricco for questioning his claim that Bell’s car never bumped Detective Isnora, saying, “It’s not rocket science, man.”

Mr. Ricco suggested that Mr. Guzman’s combative demeanor on the stand was “exactly what was going on in front of that club.”