Los Angeles - Police fatally shot a homeless man on Skid Row during a "brutal" videotaped struggle in which a rookie officer cried out that the man had grabbed his gun, the Los Angeles police chief said Monday.

Video showed the man reaching toward the officer's waistband, Chief Charlie Beck said. The officer's gun was found partly cocked and jammed with a round of ammunition in the chamber and another in the ejection port, indicating a struggle for the weapon.

"You can hear the young officer who was primarily engaged in the confrontation saying that 'He has my gun. He has my gun,' " Beck said. "He says it several times, with conviction."

Then three other officers opened fire.

The man was black, as was the rookie officer who was just short of completing his probationary year on the force, police said.

Beck's narrative of the shooting, including photos from video showing the condition of the gun, was rare, emerging just 24 hours after an officer-involved shooting. It came amid heightened attention to killings by police officers that have led to protests, some violent, across the country.

Sunday's violence had echoes of the August police shooting of 25-year-old Ezell Ford, whose death in a struggle with LA officers brought demonstrations in the city. Ford was unarmed. Police said he was shot after reaching for an officer's gun.

Mayor Eric Garcetti said he and the police chief needed to respond quickly to reassure residents that there is a robust investigation into the shooting, which occurred in the downtown area that is home to the city's highest concentration of homeless people.

"I watched the video, I watched the tragic events on Skid Row unfold," the mayor said. "We owe the city a thorough investigation as to what happened."

The shooting was caught by at least four cameras, two held by witnesses and two worn by officers who fired their weapons. There was also a camera in a police car and a security camera on a nearby shelter captured events leading up to the incident.

The American Civil Liberties Union called on the Police Department to quickly release footage shot by the officers' body cameras.

One witness posted his video to Facebook, which drew millions of views.

Police were investigating a reported robbery when they tried to talk with the suspect and he refused to obey their commands and started fighting, Beck said. Stun guns fired at the man had "appeared to have little effect and he continued to violently resist."

As the man took swings, four officers wrestled him to the ground. Two other officers subdued and handcuffed a woman who had picked up a dropped baton.

The struggle became blurry and distant, but shouting could be heard, followed by five apparent gunshots.

A memorial sprung up where the shooting occurred. White roses were placed over a tent, blankets and clothing belonging to the dead man known as "Africa."

James Attaway, 48, said the man's first name was Shawn, but he nicknamed him because he was from Africa, though he had family in Boston. They met six months ago, and Attaway said they slept near each other.

Africa had been living on the street for about a year, Attaway said. They met talking about God and had done that earlier Sunday.

"He was on the spiritual side, very intelligent," Attaway said.

Tents and cardboard shelters cover the sidewalks of Skid Row, where an estimated 1,700 homeless people live. Many of them struggle with mental illness and addiction and are no strangers to the police.

Cmdr. Andrew Smith said the man had previous encounters with officers, though he would not elaborate. Authorities withheld the man's name.

The three officers who fired their weapons were veterans of the beat and had special training to deal with the homeless and mentally ill.

"They were trained to work with homeless," said Police Commission President Steve Soboroff. "It wasn't a SWAT team looking for problems."

The shooting is being investigated by the Los Angeles Police Department's inspector general and the city's district attorney.

Activists called on Gov. Jerry Brown to appoint a special investigator to examine the killing.

Earl Ofari Hutchinson, head of the Los Angeles Urban Policy Roundtable, urged the city Police Commission to hold a special hearing on use of force by officers in Skid Row.

Two of the officers suffered minor injuries in the scuffle, including the rookie officer, who is on crutches. All four officers were placed on paid leave.