But they have been defied in many areas, with shops looted, cars burned and buildings attacked. Riot police have used tear gas and rubber bullets.

President Donald Trump urged "healing" over the death of George Floyd but said he could not allow mobs to dominate.

A white ex-policeman is charged with murdering Mr Floyd, 46, in Minneapolis.

Derek Chauvin, 44, is due to appear in court on Monday.

In video footage, Mr Chauvin can be seen kneeling on Mr Floyd's neck for several minutes on Monday. Mr Floyd repeatedly says that he is unable to breathe.

Three other officers present at the time have also since been sacked.

Speaking later in the day at Florida’s Kennedy Space Center, Trump announcing a civil rights investigation and painting himself as a supporter of those angered by Floyd’s death.

“I stand before you as a friend and ally to every American seeking justice and peace. And I stand before you in firm opposition to anyone exploiting this tragedy to loot, rob, attack and menace,” he said.

But he also again lashed out at “Antifa and the violent left” whom he blames for protests spreading across the US.

“I understand the pain that people are feeling. We support the right of peaceful protestors and we hear their pleas. But what we are now seeing on the streets of our cities has nothing to do with justice or with peace,” Trump said.

“The memory of George Floyd is being dishonored by rioters, looters and anarchists. The violence and vandalism is being led by Antifa and other radical leftwing groups who are terrorizing the innocent, destroying jobs, hurting businesses and burning down victims.

“Right now, America needs creation not destruction, cooperation not contempt, security not anarchy, and there will be no anarchy. We cannot and must not allow a small group of criminals and vandals to wreck our cities and lay waste to our communities.”

Huge demonstrations have taken place in at least 30 cities across the US. They were largely peaceful on Saturday, but violence flared later in the day.

One of the cities worst affected by unrest is Los Angeles. California Governor Gavin Newsom declared a state of emergency in the city and activated the National Guard - the reserve military force that can be called on to intervene in domestic emergencies.

The entire city is under a 20:00 to 05:30 curfew. Numerous shops have been looted, including on the famous retail avenues, Melrose and Fairfax, while overhead footage showed fires burning. Earlier police fired rubber bullets and hit protesters with batons. Hundreds of arrests have been made.

In the nation’s capital, hundreds of demonstrators assembled near the Justice Department headquarters shouting, “black lives matter.” Many later moved to the White House, where they faced off with shield-carrying police, some mounted on horseback.

Thousands flooded Chicago streets for a second day of protests. Cellphone footage shared with Reuters showed an overturned SUV, a patrol car on fire, a person burning the American flag and a skirmish between demonstrators and police.

In the Brooklyn borough of New York City, video footage recorded by onlookers showed a police squad car driving into a crowd of protesters during a second day of violence after more than 200 arrests were made on Friday.

“They could’ve killed them, & we don’t know how many they injured,” U.S. Representative Alexandria Ocasio-Cortez wrote on Twitter, demanding the NYPD officers be brought to justice.

At a late-night news conference, New York Mayor Bill de Blasio suggested protesters were to blame.

“The video was upsetting, and I wish the officers hadn’t done that,” de Blasio said. “But I also understood that they didn’t start the situation. The situation was started by a group of protesters converging on a police vehicle, attacking that vehicle.”

The mayor said flare-ups of violence were primarily instigated by small numbers of outsiders determined to goad police into confrontations, a phenomenon authorities said was playing out across the country.

Protesters in Los Angeles clashed with police in the city’s Fairfax district, where crowds tried to force their way into the CBS Television City studios but were repulsed by law enforcement and security officers, the Los Angeles Times said.

Looters broke into stores at The Grove shopping center in Fairfax, TV reported, as well as into shops along the famed Rodeo Drive of wealthy Beverly Hills.

Los Angeles Mayor Eric Garnett told reporters, “This is no longer a protest. This is vandalism ... this is destruction.”