Police pepper spray protesters during a demonstration in Minneapolis, Minnesota. AFP
The video, taken by an onlooker to Monday night's fatal encounter between police and George Floyd, 46, showed him lying face down and handcuffed, groaning for help and repeatedly saying, "please, I can't breathe," before growing motionless.
The second day of demonstrations, accompanied by looting and vandalism, began hours after Mayor Jacob Frey urged prosecutors to file criminal charges against the white policeman shown pinning Floyd to the street.
Floyd, who was unarmed and reportedly suspected of trying to pass counterfeit bills at a corner eatery, was taken by ambulance from the scene of his arrest and pronounced dead the same night at a hospital.
The policeman shown kneeling on Floyd's neck and three fellow officers involved were dismissed from the police department on Tuesday as the FBI opened an investigation.
Hundreds of protesters, many with faces covered, thronged streets around the Third Precinct police station late on Wednesday, about half a mile from where Floyd had been arrested, chanting, "No justice, no peace" and "I can't breathe."
The crowd grew to thousands as night fell and the protest turned into a standoff outside the station, where police in riot gear formed barricade lines while protesters taunted them from behind makeshift barricades of their own.
Police, some taking positions on rooftops, used tear gas, plastic bullets and concussion grenades to keep the crowds at bay. Protesters pelted police with rocks and other projectiles. Some threw tear gas canisters back at the officers.
Television news images from a helicopter over the area showed dozens of people looting a Target store, running out with clothing and shopping carts full of merchandise.
Fires erupted after dark at several businesses, including an auto parts store. Eyewitnesses said the blazes appeared to be the work of arsonists. Media said a smaller, peaceful protest was held outside the home of one of the police officers.
Outrage at Floyd's death also triggered a rally in his name against police brutality by hundreds of people in downtown Los Angeles on Wednesday afternoon.
That demonstration turned violent after a crowd marched onto a nearby freeway and blocked traffic, then attacked two California Highway Patrol cruisers, smashing their windows, local media reported. One protester who clung to the hood of a patrol car fell to the pavement as it sped away, and was treated at the scene by paramedics, news footage of the incident showed.
The video of Monday's deadly confrontation between Minneapolis police and Floyd led Mayor Frey to call on Wednesday for Hennepin County Attorney Mike Freeman "to charge the arresting officer in this case".
The city identified the four officers as Derek Chauvin, Thomas Lane, Tou Thao and J Alexander Kueng. It did not say who knelt on Floyd's neck, and gave no further information.
The local police union said the officers were cooperating with investigators and cautioned against a "rush to judgment".
"We must review all video. We must wait for the medical examiner's report," the union statement said.
The county attorney's office said it would decide how to proceed once investigators had concluded their inquiries.
The case was reminiscent of the 2014 killing of Eric Garner, an unarmed black man in New York City who died after being put in a banned police chokehold.
Garner's dying words, "I can't breathe," became a rallying cry for the Black Lives Matter movement calling attention to a wave of killings of African-Americans by police using unjustified lethal force.
The authorities in the United States need to initiate tough and serious action to stop such mindless killings and ensure that justice is done when they do occur.
Little wonder that thousands protesting racism have targeted images and place names honouring slavers and imperialist figures who perpetrated these historical outrages.
The rallies, dominated by a heavy police presence, were mostly peaceful. Protesters marched on the streets or gathered at public parks carrying posters that said "No Justice, No Peace" and "Sorry For The Inconvenience, We Are Trying To Change The World."
George Floyd's death sparked nationwide protests demanding a reckoning with institutional racism that have sometimes resulted in clashes with police, but many officers took a less aggressive stance over the weekend when demonstrations were overwhelmingly peaceful.
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