Memphis (AFP) - The southern US city of Memphis braced itself for unrest Friday as authorities prepared to release a video depicting the fatal assault of a Black man by five police officers who, the victim's mother said, "beat him to a pulp."

The police officers, who are also Black, were charged with second-degree murder in the beating of 29-year-old Tyre Nichols, who died in hospital on January 10, three days after being stopped on suspicion of reckless driving.

"They had beat him to a pulp," Nichols's mother RowVaughn Wells told CNN, sobbing as she described him in hospital. "He had bruises all over. His head was swollen like a watermelon. His neck was bursting because of the swelling."

"I knew my son was gone," Wells said.

Memphis police chief Cerelyn Davis said the graphic video, which will be released after 6:00 pm Central time (0000 GMT Saturday), shows Nichols crying out for his mother.

"What I saw on this video was more of a groupthink sort of mentality. And no one took a step to intercept or intervene," Davis said. "And that's why the charges are as severe as they are."

Davis compared the video to footage of the 1991 Rodney King beating, which sparked days of riots in Los Angeles that left dozens dead.

"I was in law enforcement during the Rodney King incident, it's very much aligned with that same type of behavior," Davis said. "I would say it's about the same, if not worse."

Police brutality

Nichols's death at the hands of police drew immediate comparisons with the May 2020 killing of George Floyd, another Black man whose suffocation by a white police officer in Minneapolis was caught on film.

Video of Floyd's death spread rapidly, sparking a massive wave of at times violent protests nationwide and beyond, and reviving scrutiny of race relations and a culture of police brutality in the United States.

President Joe Biden, anticipating protests after the Memphis video's release, called for calm, saying "outrage is understandable, but violence is never acceptable."

The police officers were taken into custody following a rapid internal investigation that found them to have deployed excessive use of force and to have failed to render aid.

In addition to second-degree murder charges, the officers are also facing indictments for aggravated assault and aggravated kidnapping.

Four of the five were released from jail after posting bail, US media reported Friday, citing jail records.

Police officer Derek Chauvin was convicted of murder over Floyd's death, in what was seen as a landmark case.

Chauvin, a white veteran of the Minneapolis police force, knelt on Floyd's neck for nearly 10 minutes, indifferent to his cries and the warnings of distraught passersby.

In another of a long series of police killings, an officer was found guilty in 2021 of manslaughter after she shot dead young African American man Daunte Wright, claiming she mistook her gun for her taser during a traffic stop in a suburb of Minneapolis.

Murder charge

RowVaughn Wells on Friday accused police of initially trying to cover-up her son's beating, coming to her door to say he had been arrested for drunk driving and pepper-sprayed and tasered after being hard to handcuff.

They said he was being treated by paramedics and would be taken to hospital but that she could not visit him.

Doctors called days later to say he was in cardiac arrest and his kidneys were failing, and that she needed to visit as soon as possible.

"People don't know what those Black police officers did to our family," Wells said.

"They have brought shame to their own families. They brought shame to the Black community."

A lawyer for one officer, Desmond Mills, said his client was innocent of second-degree murder.

"That requires that they prove that Mr Mills acted with a reasonable degree of certainty with regard to his actions, that his actions were certain to cause death. And that's just simply not the case," said Blake Ballin.