The mayor of Atlanta ordered immediate police reforms on Monday after the fatal shooting of a black man by a white police officer in the US city sparked further outrage over the deaths of African-Americans at the hands of law enforcement.

"Our police officers are to be guardians and not warriors in our communities," Atlanta Mayor Keisha Lance Bottoms told a press conference.

This undated handout photo obtained June 14, 2020, courtesy of the Atlanta Police Department, shows Atlanta police officer Garrett Rolfe This undated handout photo obtained June 14, 2020, courtesy of the Atlanta Police Department, shows Atlanta police officer Garrett Rolfe Photo: The Atlanta Police Department / Handout

Bottoms described Friday night's police shooting of 27-year-old Rayshard Brooks as a "murder" and said it "didn't have to end that way."

"It angered me and it saddened me beyond words," the mayor said.

Brooks's death came two-and-a-half weeks after the death of George Floyd, a black man killed by a white police officer in Minneapolis. Floyd's May 25 death ignited a wave of protests for racial justice and police reform across the United States. It also triggered a national soul-searching of the country's checkered past regarding racism.

A woman walks past a makeshift memorial to George Floyd near the site where he died in police custody in Minneapolis, Minnesota A woman walks past a makeshift memorial to George Floyd near the site where he died in police custody in Minneapolis, Minnesota Photo: AFP / Kerem Yucel

President Donald Trump is due to sign an executive order Tuesday to encourage "best practices" among the police, but the move will likely fall short of calls for a fundamental shake-up of the service.

Bottoms outlined reforms to standard operating procedures including use of deadly force policies and a "duty to intervene" if an officer sees misconduct by another policeman.

The death of 27-year-old Rayshard Brooks was ruled a homicide by the county medical examiner's office on Sunday, a day after the Wendy's restaurant where he died was set on fire and hundreds of people marched to denounce the killing The death of 27-year-old Rayshard Brooks was ruled a homicide by the county medical examiner's office on Sunday, a day after the Wendy's restaurant where he died was set on fire and hundreds of people marched to denounce the killing Photo: AFP / Elijah Nouvelage

"This is the beginning of a great deal of work that lies ahead of us," she said. "It is clear that we do not have another day, another minute, another hour to waste."

The mayor's address came after thousands of demonstrators marched on the Georgia state capitol in Atlanta to protest Brooks's death.

Georgia prosecutors said they were weighing bringing charges against the policeman who shot Brooks in a Wendy's parking lot.

Atlanta residents gather at site where Rayshard Brooks was killed Atlanta residents gather at site where Rayshard Brooks was killed Photo: AFPTV / Wes BRUER

"If that shot was fired for some reason other than to save that officer's life or to prevent injury to him or others, then that shooting is not justified under the law," Fulton County district attorney Paul Howard said.

The officer, Garrett Rolfe, was dismissed from the force, and Atlanta's police chief resigned within hours of the shooting.

A man holds a sign outside the Wendy's restaurant where Rayshard Brooks was shot dead by police A man holds a sign outside the Wendy's restaurant where Rayshard Brooks was shot dead by police Photo: AFP / Elijah Nouvelage

Trump described the Atlanta incident as "very disturbing."

Protestors gather for a march organized by the National Association for the Advancement of Colored People (NAACP) in Atlanta, Georgia, to protest the fatal shooting of a black man by a white police officer Protestors gather for a march organized by the National Association for the Advancement of Colored People (NAACP) in Atlanta, Georgia, to protest the fatal shooting of a black man by a white police officer Photo: AFP / Wes BRUER

"It's about law and order but it's about justice also, and it's about safety," he said ahead of unveiling the executive order on police standards.

Trump has condemned Floyd's death but has rejected accusations of systemic racism in the police force.

Atlanta Mayor Keisha Lance Bottoms ordered a series of police reforms Atlanta Mayor Keisha Lance Bottoms ordered a series of police reforms Photo: GETTY IMAGES / Paras Griffin

Tomika Miller, Brooks's wife, told "CBS This Morning" that Rolfe and the other officer who was on the scene should go to jail.

"If it was my husband who shot them, he would be in jail," Miller said. "They need to be put away."

Brooks was shot while running away after a scuffle with the two officers, which was caught on video. The autopsy report said he died of two gunshot wounds in the back.

Thousands of demonstrators chanting "Black lives matter" took part in the march on the capitol, where the Georgia legislature was holding its first session since the coronavirus pandemic.

The march was called by the state chapter of the civil rights group the National Association for the Advancement of Colored People.

"We are done dying," the NAACP said in a statement that called for "ending police violence against our communities."

The demonstrators held up photos of Floyd, Brooks and another Georgian, Ahmaud Arbery, a black jogger shot dead on February 23 after being chased down by three white men in pickup trucks.

Lloyd Pierce, the head coach of the local National Basketball Association team the Atlanta Hawks, addressed the crowd.

"I was born a black man," Pierce said. "And I know one day I'll die a black man. But I don't want to die because I'm a black man."

Police reform has been one of the most persistent demands of protestors who have taken to the streets of US cities. Chicago's mayor Lori Lightfoot announced on Monday that she was setting up a working group to review the use of force by officers.

In California, police unions in San Jose, San Francisco and Los Angeles condemned Floyd's death and pledged to reform their policies on use of force and root out racist officers.

In New York, police commissioner Dermot Shea said about 600 plainclothes police officers would be shifted to new roles including "community policing."

"This is a seismic shift in the culture of how the NYPD polices this great city," Shea said.

Brooks's death came after police responded to a complaint that a man was asleep in his car at the Wendy's.

Brooks was shot while running away after grabbing one of the officer's Taser stun devices. Video appeared to show him turning and pointing the Taser just before the shooting.