A black man who died in police custody in Minneapolis was seen on a bystander's video pleading that he could not breathe as a white officer knelt on his neck during the arrest.
The officer kept his knee there for several minutes after the man stopped moving.
Four officers were fired after the death on Monday night after a struggle with officers and the incident was under investigation by the FBI and state law enforcement authorities.
It drew comparisons to the case of Eric Garner, an unarmed black man who died in 2014 in New York after he was placed in a chokehold by police and pleaded for his life, saying he could not breathe.
Minneapolis Mayor Jacob Frey apologised to the black community Tuesday in a post on his Facebook page.
"Being Black in America should not be a death sentence. For five minutes, we watched a white officer press his knee into a Black man's neck. Five minutes. When you hear someone calling for help, you're supposed to help. This officer failed in the most basic, human sense," Frey posted.
Police said the man matched the description of a suspect in a forgery case and resisted arrest. The unidentified officer ignores his pleas. "Please, please, please, I can't breathe. Please, man," the man is heard telling the officer.
After several minutes, one of the officers tells the man to "relax". Minutes pass, and the man becomes motionless under the officer's restraint. The officer leaves his knee on the man's neck for several minutes more.
Several witnesses had gathered on a nearby footpath, some recording the scene on their phones. The bystanders became increasingly agitated as the man pleaded with police. One bystander told officers they needed to let him breathe. Another yelled at them to check the man's pulse.
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The man who died was identified as George Floyd by Ben Crump, a prominent civil rights and personal injury attorney who said he had been hired by Floyd's family.
"We all watched the horrific death of George Floyd on video as witnesses begged the police officer to take him into the police car and get off his neck," Crump said. "This abusive, excessive and inhumane use of force cost the life of a man who was being detained by the police for questioning about a non-violent charge."
Charles McMillian, 60, of Minneapolis, said he saw police trying to get Floyd into the back of the squad car and heard Floyd tell them he was claustrophobic.
After having the officer's knee on his neck, McMillian said, the man started calling his mother's name, "and then he died".
"It's sad because it didn't have to happen," McMillian said.
Minneapolis Police Chief Medaria Arradondo said the department would conduct a full internal investigation but four officers were later fired.
Minneapolis Mayor Jacob Frey announced the terminations on Twitter, and said the firings were the "right call."
Experts on police use of force told AP that the officer clearly restrained the man too long. They noted the man was under control and no longer fighting. Andrew Scott, a former Boca Raton, Florida, police chief who now testifies as an expert witness in use-of-force cases, called Floyd's death "a combination of not being trained properly or disregarding their training".
"He couldn't move. He was telling them he couldn't breathe, and they ignored him," Scott said. "I can't even describe it. It was difficult to watch."
The New York City officer in the Garner case said he was using a legal manoeuvre called "the seatbelt" to bring down Garner, who police said had been resisting arrest. But the medical examiner referred to it as a chokehold in the autopsy report and said it contributed to his death. Chokehold manoeuvres are banned under New York police policy.
A grand jury later decided against indicting the officers involved in Garner's death, sparking protests around the country.
In Minneapolis, kneeling on a suspect's neck is allowed under the department's use-of-force policy for officers who have received training in how to compress a neck without applying direct pressure to the airway. It is considered a "non-deadly force option", according to the department's policy handbook.
A chokehold is considered a deadly force option and involves obstructing the airway. According to the department's use-of-force policy, officers are to use only an amount of force necessary that would be objectively reasonable.
The police union asked the public to wait for the investigation to take its course and not to "rush to judgment and immediately condemn our officers". The Hennepin County Attorney's Office, which would handle any prosecution of police on state charges, said it was "shocked and saddened" by the video and pledged to handle the case fairly. The US Attorney's Office in Minnesota declined to comment.
The death came amid outrage over the death of Ahmaud Arbery, a black man who was fatally shot on February 23 in Georgia after a white father and son pursued the 25-year-old they had spotted running in their subdivision. More than two months passed before charges were brought. Crump also represents Arbery's father.
All body camera footage in the Minneapolis case was turned over to the Minnesota Bureau of Criminal Apprehension, and the agency asked to speak with anyone who saw the arrest or recorded video. The officers involved have been put on paid administrative leave, per department protocol. The agency said the officers' names will be released after initial interviews with the people involved and witnesses.
The FBI is conducting a separate federal civil rights investigation, at the request of Minneapolis police, the BCA said. Messages left with the FBI were not immediately returned.
The American Civil Liberties Union was among many organisations calling for the officers to be held accountable.
"You can't watch this appalling video posted by brave eyewitnesses on social media without seeing police officers' callous disregard for a black man's life," ACLU of Minnesota executive director John Gordon said, calling the death "needless and preventable".