Police cars were set ablaze and there were reports of injuries mounting on all sides, reports AP.

Triggered by the death of Floyd in Minneapolis on Monday after a police officer kneeled on his neck, the unrest has since become a national phenomenon with protesters decrying years of deaths at police hands.

But the large crowd protesters, many not wearing masks or maintaining social distancing, health experts fear it could further spread coronavirus infection as the country emerges from months in lockdown.

After a tumultuous Friday night, racially diverse crowds took to the streets again for mostly peaceful demonstrations in dozens of cities from coast to coast. The previous day's protests also started calmly, but many descended into violence later in the day.

"Our country has a sickness. We have to be out here," said Brianna Petrisko, among those at lower Manhattan's Foley Square, where most were wearing masks amid the coronavirus pandemic. "This is the only way we're going to be heard."

In Minneapolis, 29-year-old Sam Allkija said the damage seen in recent days reflects longstanding frustration and rage in the black community. "I don't condone them," he said. "But you have to look deeper into why these riots are happening."

Minnesota Gov Tim Walz fully mobilised the state's National Guard and promised a massive show of force.

"The situation in Minneapolis is no longer in any way about the murder of George Floyd," Walz said. "It is about attacking civil society, instilling fear and disrupting our great cities."

More than a dozen major cities nationwide imposed overnight curfews. People were also told to be off the streets of Atlanta, Denver, Los Angeles, Seattle and Minneapolis.

The unrest comes at a time when most Americans have spent months in lockdown. Hundreds of people were arrested Friday, and police used batons, rubber bullets and pepper spray on crowds in some cities. Many departments reported injured officers, while social media platforms were awash in images of police using forceful tactics, throwing people to the ground, using bicycles as shields and in one instance trampling a protester while on horseback.

Authorities vowed to crack down on lawbreakers.

"Quite frankly, I'm ready to just lock people up," Atlanta Police Chief Erika Shields said at a news conference.

This week's unrest recalled the riots in Los Angeles nearly 30 years ago after the acquittal of the white police officers who beat Rodney King, a black motorist who had led them on a high-speed chase.

The protests of Floyd's killing have gripped many more cities, but the losses in Minneapolis have yet to approach the staggering totals Los Angeles saw during five days of rioting in 1992, when over 60 people died, 2,000-plus were injured and thousands arrested, with property damage topping $1 billion.

Many protesters spoke of frustration that Floyd's death was one more in a litany. It came in the wake of the killing in Georgia of Ahmaud Arbery and in the middle of the coronavirus pandemic that has thrown millions out of work, killed more than 100,000 people in the US and disproportionately affected black people.

The officer who held his knee to Floyd's neck was arrested Friday and charged with third-degree murder and second-degree manslaughter. But many protesters are demanding the arrests of the three other officers involved.

Trump stoked the anger, firing off a series of tweets criticising Minnesota's response, ridiculing people who protested outside the White House and warning that if protesters had breached its fence they would "have been greeted with the most vicious dogs, and most ominous weapons, I have ever seen."