MINNEAPOLIS, May 30 ― A white former Minneapolis police officer was charged with murder yesterday after a bystander video showed him pinning his knee into the neck of an unarmed black man who later died, an incident that triggered three nights of violent protests.

Derek Chauvin, the officer seen on the cellphone video footage kneeling on George Floyd's neck, is in custody and has been charged with third-degree murder and manslaughter, Hennepin County Attorney Mike Freeman told a news briefing.

"We have evidence, we have the citizen's camera's video, the horrible, horrific, terrible thing we have all seen over and over again, we have the officer's body-worn camera, we have statements from some witnesses,” Freeman said.

The graphic footage, taken during the Monday evening arrest, shows Floyd, 46, gasping for air and pleading for help as he repeatedly groans, "please, I can't breathe,” while onlookers can be heard shouting at police to let him up.

Floyd gradually grew unresponsive and ceased to move. He was pronounced dead at a local hospital a short time later.

Chauvin had his knee on Floyd's neck for 8 minutes and 46 seconds, according to court documents. An autopsy found the combined impact of being restrained by police, underlying health conditions and any potential intoxicants in his system likely contributed to his death.

The charges came after a third night of turmoil, which saw protesters set fire to a police station in Minneapolis and the National Guard deployed to help restore order.

Minneapolis Mayor Jacob Frey imposed an 8pm curfew to try to stop the violence that has wracked the city the last three nights. The curfew will extend through the weekend, expiring at 6am each morning, and prohibits people from traveling on public streets or gathering in a public place.

Floyd, a Houston native who had worked security for a nightclub, was arrested for allegedly using counterfeit money at a store to buy cigarettes on Monday evening. An employee who called police described the suspect as possibly drunk, according to an official transcript of the call.

Chauvin and three other officers at the scene were fired on Tuesday from the Minneapolis Police Department. The city has identified them as Thomas Lane, Tou Thao and J Alexander Kueng.

Freeman said the investigation into Chauvin, who faces up to 25 years in prison if he is convicted, was continuing and he anticipated filing charges against the other officers as well. He said it was appropriate to charge "the most dangerous perpetrator” first.

‘Harrowing’

US Attorney General William Barr called the video "harrowing and deeply disturbing” in a written statement and said a parallel federal investigation would determine if the officers violated civil rights laws.

Floyd's death recalled the 2014 killing of Eric Garner, an unarmed black man in New York City, who died after being put in a police chokehold and telling the officers, "I can't breathe.”

Mike Griffin, a community organiser in Minneapolis, said the resulting protests reflected years of frustration over economic inequalities and the feeling that black lives were not valued as highly by police.

He said anger has been building since the 2015 fatal shooting by Minneapolis police of 24-year-old Jamar Clark, and the 2016 killing of Philando Castile, a 32-year old black man shot by Minnesota police during a traffic stop.

"George's murder was just the last straw,” Griffin told Reuters, calling the charges against Chauvin a step in the right direction, but adding there needed to be systemic changes.

The protests, which threatened to carry into a fourth night, have been driven in part by the initial lack of arrests in the case.

Responding to a reporter's question about why no officer was arrested sooner, Hennepin County Attorney Freeman stressed that charges in similar cases typically take nine months to a year.

"This is by far the fastest we've ever charged a police officer,” said Freeman.

Earlier yesterday, Minnesota governor Tim Walz said he understood why people had "lost faith” in the police but argued Floyd's plight had gotten "lost in 48 hours of anarchy” and called for an end to destruction of property. He has declared a state of emergency and called in the state's National Guard.

"None of us can live in a society where roving bands go unchecked and do what they want to, ruin property,” Walz said, adding that a reckoning with the city's racial inequities could not happen without calm. "We have to get back to that point of what caused this all to happen and start working on that.”

The governor also apologised to CNN after police arrested three members of its TV crew during an early morning sweep.

Protesters also took to the streets of several other US cities yesterday over the killing of Floyd and other unarmed black people, including Breonna Taylor, who was fatally shot by police in her home in Louisville, Kentucky, in March. ― Reuters