One spring afternoon two years ago, a young undercover police officer was waiting nervously in a labyrinthine mini-storage building in Chelsea, watching over a stash of counterfeit CD's until his superiors returned.

Suddenly, he came upon an unarmed man who spoke little English. The man, an immigrant from West Africa, and the officer began to struggle. The man did not stop and grabbed for the officer's gun, the officer said, even after the officer identified himself.

Several minutes later, after a short chase down a dead-end corridor, the police said, the officer fired his gun four times at the man, hitting him in the chest and abdomen, killing him.

The officer, Bryan A. Conroy, goes on trial today in the May 22, 2003, killing of the immigrant, Ousmane Zongo. Officer Conroy has pleaded not guilty to reckless manslaughter.

His lawyer, Stuart London, faces a difficult task. He will try to convince a jury that the shooting was justified, because his client, on police duty, was trying to arrest Mr. Zongo. But prosecutors say that it was Officer Conroy who started the struggle, and that a stricter standard should apply.

It is the first trial of a New York City officer in the death of a civilian since the trial in 2000 of four officers who killed another unarmed African immigrant, Amadou Diallo, in 1999. Mr. Diallo was shot 19 times in the vestibule of his Bronx apartment building, in an episode that put the Police Department under criticism for what some said was overly aggressive policing, and led to a change in some of its tactics.

But convictions in police killings of civilians are rare. In the case of Mr. Diallo, jurors acquitted the police officers involved of murder, saying that prosecutors had failed to prove they acted criminally. Last year, a grand jury refused to indict a police officer for accidentally shooting and killing a teenager while on patrol on a Brooklyn rooftop.

Mr. Zongo, 43, arrived from Burkina Faso in 2001, and worked restoring African art, which was stored in the Chelsea warehouse.

The central issue at trial will be whether Officer Conroy was trying to arrest Mr. Zongo, an action that could lead a jury to conclude the shooting was justified. Officer Conroy's lawyer, Mr. London, contends that his client was trying to arrest Mr. Zongo, who resisted, even knocking Officer Conroy's nose with his shoulder when he tried to grab the officer's gun.

Prosecutors say, however, that Officer Conroy had no reason to believe Mr. Zongo was a threat, and that he, in fact, acted first, aiming his loaded gun at Mr. Zongo. Prosecutors argue that Officer Conroy should be treated as a civilian and should not be given the legal leeway allowed when a police officer is making an arrest.

The circumstances could not have been worse. The warehouse, where police had executed a search warrant that day as part of a counterfeiting investigation, was dark, and Officer Conroy had been left without backup. He was unfamiliar with the layout, and did not know that the warehouse had not been closed to civilians. Court papers citing remarks Officer Conroy was said to have made to colleagues, once they returned, evoke a frightened rookie.

"What took you so long? This guy wouldn't stop fighting," Officer Conroy said, according to court papers. "He kept coming and coming; he tried to get my gun."

Officer Conroy, who is 26, joined the police force in September 2000.

Mr. London argues that accusing the officer of being the aggressor is unfair. Officer Conroy, though undercover, was on active duty at the time of the shooting, and believed Mr. Zongo was a threat.

"It would have a chilling effect on all undercover operations," Mr. London said yesterday.

Officer Conroy arrived at a hearing in State Supreme Court in Manhattan yesterday dressed sharply in a pinstriped suit with a small American flag button on his lapel. His parents and his wife, whom he married in the last year, came with him.

Since the shooting, the officer has kept a low profile, Mr. London said. He has continued to work for the police department monitoring video screens in Staten Island, where he lives. He goes to the gym to work out, and spends time with his new wife.

Also in court were Mr. Zongo's widow, Salimata Sanfo, and two of his brothers, who had made the trip from their 3,000-person village, Zogbega Yako, to attend the trial. Mr. Zongo's family has filed a $150 million lawsuit against the city, claiming wrongful death.

Outside the courthouse yesterday, two interpreters turned the tribal language spoken by Ms. Sanfo into English before the family was whisked away in a black limousine with their lawyer, Sanford Rubenstein.

"She said that when she saw the policeman for the first time, she was mad, she was ready to yell and cry," said Mohamed Dibassy, one of the interpreters.