An unarmed groom-to-be ignored multiple warnings and used his car to “assault” an undercover detective before police killed him in a hail of 50 bullets, an officer testified yesterday.

Taking the witness stand at the trial of three undercover detectives charged in the slaying of Sean Bell, Officer Michael Carey testified that amid the chaos of flying bullets and broken glass, he believed “the passenger of the car was firing at us.”

He also insisted that one of the defendants, Detective Gescard Isnora, “was yelling, ‘Police! Show your hands! Show your hands!'” The victims contend that the officers did not identify themselves. Mr. Carey, who was the defense’s first witness, was also the first shooter to give a public account of the shooting of Bell on November 25, 2006 — his wedding day — outside a Queens topless bar where Bell had his bachelor party.

Detective Isnora and Detective Michael Oliver have pleaded not guilty to manslaughter, and Detective Marc Cooper has pleaded not guilty to reckless endangerment. Mr. Carey fired his gun three times, but wasn’t charged.

The defendants have claimed through their lawyers that the shooting was justified in part because they overheard Bell’s friend, Joseph Guzman, threaten to retrieve a weapon and retaliate.

Mr. Guzman and another man seriously injured in the shooting testified earlier this week that nothing was amiss that night, and that the three men were unaware that police were shadowing them. As they got into Bell’s car to drive home, a man with a gun “appeared out of nowhere” and without warning began shooting like he was “out of his mind,” Mr. Guzman said.

In a somber monotone, Mr. Carey, 27, stuck to a starkly different scenario offered by police. He said he was riding in an unmarked police van driven by Detective Oliver when they were radioed that Detective Isnora was tailing a possible armed man and needed backup. When the van pulled onto the block where Bell’s car was parked, Mr. Carey spotted Detective Isnora approaching the car with his gun drawn and his voice raised. “Are you certain you heard commands?” the defense attorney, Anthony Ricco, said.

“Not a doubt,” the witness responded.

Mr. Carey said over the next few seconds, he witnessed Bell’s car race forward, bump Detective Isnora, and smash into the front of the van. The driver then backed into a security gate before gunning the engine again and heading toward the detective.

At the same time, he heard Detective Isnora shout, “He’s got a gun! He’s got a gun!” and begin shooting at the front-seat passenger — Mr. Guzman.

Believing the passenger was armed and seeing the driver “assault a police officer” with the car, Mr. Carey said he decided to hop out of the van and use deadly force himself. He described approaching the car when the shooting stopped and, seeing Bell and Mr. Guzman motionless, thinking both were dead.

On cross-examination, Mr. Carey admitted hearing his dazed team talk about how no gun had been found. He also testified that a police union delegate quickly dragged him away from the scene, but denied that they talked about what had happened.

The delegate “specifically didn’t ask me” about the shooting, he said.