A demonstrator runs to kick a crowd-control canister during protests in Oakland, California, US on Friday. Reuters

As unrest spread across dozens of American cities on Friday, the Pentagon took the rare step of ordering the Army to put several active-duty US military police units on the ready to deploy to Minneapolis, where the police killing of George Floyd sparked the widespread protests.

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Soldiers from Fort Bragg in North Carolina and Fort Drum in New York have been ordered to be ready to deploy within four hours if called, according to three people with direct knowledge of the orders. Soldiers in Fort Carson, in Colorado, and Fort Riley in Kansas have been told to be ready within 24 hours. The people did not want their names used because they were not authorised to discuss the preparations.

The get-ready orders were sent verbally on Friday, after President Donald Trump asked Defence Secretary Mark Esper for military options to help quell the unrest in Minneapolis after protesters descended into looting and arson in some parts of the city.

A demonstrator kicks a tear gas canister that was deployed by police officers during protests in Oakland, California. AFP

The white former Minneapolis police officer shown in video footage using his knee to pin an unarmed black man's neck to the street was charged with murder on Friday in the man's death, an incident that has unleashed four nights of violent protests.

Derek Chauvin, who was dismissed from the police department with three fellow officers the day after the fatal encounter, was arrested on third-degree murder and manslaughter charges for his role in Monday's death of 46-year-old George Floyd.

Graphic video footage taken by an onlooker's cell phone and widely circulated on the internet shows Floyd – with Chauvin's knee pressed into his neck – gasping for air and repeatedly groaning, "Please, I can't breathe," while a crowd of bystanders shouted at police to let him up.

After several minutes, Floyd gradually grows unresponsive and ceases to move. He was pronounced dead at a local hospital a short time later.

Flames erupt from a New York City Police Department van set ablaze during protests over the death of George Floyd. AP

The video reignited an outpouring of rage that civil rights activists said has long simmered in Minneapolis and cities across the country over persistent racial bias in the US criminal justice system.

The charges brought by Hennepin County prosecutors came after a third night of arson, looting and vandalism in which protesters set fire to a police station, and the National Guard was deployed to help restore order in Minnesota's largest city.

Authorities had hoped Chauvin's arrest would allay public anger and avert continued unrest. But defying an 8pm curfew imposed by Mayor Jacob Frey, about 500 demonstrators clashed anew on Friday evening with riot police outside the battered Third Precinct building.

Police, creating a two-block buffer area around the precinct house, opened fire with tear gas, plastic bullets and concussion grenades, scattering the crowd.

Another group of protesters later converged near the city's Fifth Precinct station until police arrived and fired tear gas and plastic bullets to break up that gathering. A nearby bank and post office were set on fire.

Police officers stand in a fog of tear gas during protests sparked by the death of George Floyd in Oakland, California. AFP

Still, Friday night's crowds were far smaller and more widely dispersed than the night before. Law enforcement kept a mostly low profile, a strategy seemingly calculated to reduce the risk of violent confrontations, as was the case in several urban centres across the country where sympathy protests arose.

Among the cities with larger protests on Friday were Los Angeles, Denver, Houston, Atlanta, Detroit and Louisville, Kentucky.

Police and Secret Service agents were out in force around the White House before dozens of protesters gathered across the street in Lafayette Square chanting, "I can't breathe."

Thousands of chanting demonstrators filled the streets of New York City's Brooklyn borough near the Barclays Center indoor arena. Police armed with batons and pepper spray made scores of arrests in sometimes violent clashes.