Curfews were imposed on major US cities as clashes over police brutality escalated across America with demonstrators ignoring warnings from President Donald Trump that his government would stop the violent protests "cold."

Minneapolis, the epicenter of the unrest, was gripped by a fifth consecutive night of violence on Saturday with police in riot gear firing tear gas and stun grenades at protesters venting fury at the death of George Floyd, an unarmed black man, during an arrest in the city on Monday.

Protesters hold up their fists as flames rise behind them
Protesters hold up their fists as flames rise behind them in Minneapolis, Minnesota, during a protest over the death of African American George Floyd in police custody. Photo: AFP / Kerem Yucel

Los Angeles, Chicago and Atlanta were among two dozen cities ordering people to stay indoors overnight as more states called in National Guard soldiers to help control the civil unrest not seen in the United States for years.

From Seattle to New York, tens of thousands of protesters took to the streets demanding tougher murder charges and more arrests over the death of Floyd, who stopped breathing after Minneapolis police officer Derek Chauvin knelt on his neck for nearly nine minutes.

Protesters call for justice for George Floyd
Protesters gather in a call for justice for George Floyd, a black man killed in Minneapolis police custody. Photo: AFP / Kerem Yucel

In Los Angeles, officers fired rubber bullets and swung batons during a testy standoff with demonstrators who set fire to a police car.

Police and protesters clashed in numerous cities including Chicago and New York, with officers responding to projectiles with pepper spray while shop windows were smashed in Philadelphia.

NYPD officers arrest protesters
NYPD officers arrest protestors during a "Black Lives Matter" demonstration in New York City. Photo: AFP / Johannes EISELE

Trump blamed the extreme left for the violence, including widespread looting and arson in Minneapolis, saying rioters were dishonoring the memory of Floyd.

"We cannot and must not allow a small group of criminals and vandals to wreck our cities and lay waste to our communities," the president said.

National Guard soldiers are deployed in Minneapolis
IMAGES National Guard soldiers are deployed in Minneapolis for peacekeeping after a third night of rioting over police brutality against African Americans left hundreds of shops damaged and a police station on fire. Photo: AFPTV / Gilles CLARENNE

"My administration will stop mob violence. And we'll stop it cold," he added, accusing the loose-knit militant anti-fascist network Antifa of orchestrating the violence.

Democratic candidate Joe Biden condemned the violence of the protests, but said on Sunday that US citizens had every right to demonstrate.

"Protesting such brutality is right and necessary," he said. "But burning down communities and needless destruction is not."

A woman brings flowers to a memorial for George Floyd
A woman brings flowers to a memorial for George Floyd, who died while in custody of the Minneapolis police, following a night of rioting. Photo: AFP / Kerem Yucel

Peaceful protests occurred too, including in Toronto as the movement spread beyond America's borders.

Demonstrators nationwide chanted slogans such as "Black Lives Matter" and "I can't breathe," which Floyd, who has become a fresh symbol of police brutality, was heard saying repeatedly before he died.

People help clean up bank and businesses in Minneapolis
People help clean up bank and businesses in Minneapolis after a fresh night of violence sparked by the death of George Floyd in police hands overwhelmed law enforcement. Photo: AFPTV / Gilles CLARENNE

"We're not turning the cheek anymore. Black lives matter. They will always matter. And we're here today to show that," said makeup artist Melissa Mock, who joined several thousand in a daytime protest in Miami.

Minnesota Governor Tim Walzs said he was mobilising the state's entire 13,000-strong National Guard to deal with rioters who have looted shops and set fires in the Minneapolis-St. Paul area.

All major freeways leading into Minneapolis were closed Saturday night with military helicopters overhead as the state braced for more rioting, arson and looting, with locals saying much of the violence was being perpetrated by outsiders.

Earlier, people congregated and chanted peacefully in Minneapolis, carrying brooms to help clean up damaged shops and streets.

Some placed flowers in front of the shop where Floyd was arrested on Monday, before his death in the hands of police was recorded in a horrifying cellphone video since seen around the world.

In Houston -- where Floyd was born and raised -- an old friend of his, Sam Osborne, said as an African American he feared for his life.

"I'm really messed up they killed him up. I'm wondering like, what could possibly happen to me?" he told AFP.

Houston's mayor announced at a press conference that Floyd's body would be brought back to the Texas city.

At least eight states -- including Texas, Colorado and Georgia -- activated the National Guard, who were also deployed around the White House to help handle the protests there.

In Washington, protesters faced off with secret service agents outside the White House for a second straight night as Trump faces the most serious spate of civil unrest of his presidency, in the middle of the coronavirus pandemic.

Looting occurred in Miami, where a curfew was also announced, while in New York mayor Bill de Blasio said a video appearing to show an NYPD police car drive into protesters in Brooklyn was "upsetting" but that he did not blame the officers.

In Los Angeles, the city's mayor expanded a curfew order as looting broke out. Stretched emergency services scrambled to put out the two blazes on Melrose Avenue, as similar scenes played out in Washington with officials extinguishing a major fire at a hotel off Layfayette Square.

Floyd's family and many protestors want a tougher charge brought and have also demanded that three officers who assisted him be charged as well.

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