• Video report by ITV News Washington Correspondent Robert Moore

Riots continue to spread across the US following the death of George Floyd, who died after a white police officer pressed his knee into his neck while taking him into custody.

Mr Floyd died after being arrested in Minneapolis - sparking four nights of protests and rioting in the city about the police's handling of the situation.

Protests have turned violent in American cities and the US state of Georgia has declared a state of emergency and activated the National Guard.

In a widely-viewed video, a white police officer can be seen pressing a knee into the neck of Mr Floyd, who was black, while taking him into custody in Minneapolis.

Protests over the death of George Floyd have erupted in cities across the US, including Los Angeles Credit: AP

After another night of watching fires burn and businesses ransacked, Minnesota’s governor Tim Walz said he is moving to activate more than 1,000 extra National Guard troops, and is considering federal help.

Tens of thousands of protesters have taken to the streets across cities in the US, including in New York, Los Angeles, Detroit, Atlanta, Washington DC and others.

Amid the chaos on the streets, a protester was shot and killed just before midnight after someone from a vehicle fired shots into a crowd in downtown Detroit.

The Guard is also on stand-by in the District of Columbia, where a crowd grew outside the White House and chanted curses at US president Donald Trump.

The White House was placed on lockdown temporarily and President Trump tweeted any protesters who got "out of line" would be "greeted with the most vicious dogs, and most ominous weapons."

Some protesters tried to push through barriers set up by the US Secret Service along Pennsylvania Avenue, and threw bottles and other objects at officers wearing riot gear, who responded with pepper spray.

In Portland, Oregon, protesters broke into the police headquarters and set fire to the building. In Virginia, a police vehicle was set alight.

In Virginia’s capital, a police cruiser was set on fire outside Richmond police headquarters, and a city transit spokeswoman said a bus set ablaze was “a total loss”, news outlets reported.

Georgia’s governor Brian Kemp tweeted that up to 500 members of the Guard would deploy immediately “to protect people & property in Atlanta”.

He said he acted at the request of Atlanta mayor Keisha Lance Bottoms, who earlier appealed in vain for calm.

In scenes both peaceful and violent across America, thousands of protesters chanted: “No justice, no peace”, and: “Say his name. George Floyd.”

They also hoisted signs, reading: “He said I can’t breathe. Justice for George.”

Police officers walk enveloped by teargas in Portland. Credit: AP

Protesters smashed windows at CNN headquarters in Atlanta, torched a police car and struck officers with bottles.

At least three officers were hurt and there were multiple arrests, Atlanta police spokesman Carlos Campos said, as protesters shot at officers with BB guns and threw bricks, bottles and knives.

While large demonstrations in New York, Houston, Washington DC, and other cities ranged from people peacefully blocking roads to clashing with police.

Atlanta officials said crews were unable to reach a fire at Del Frisco’s restaurant in the Buckhead area several miles north because of protesters there.

Protesters shattered glass at the CNN building in Atlanta. Credit: AP

At a press conference, Atlanta mayor Ms Bottoms said: “This is not in the spirit of Martin Luther King, Jr.

“You are disgracing the life of George Floyd and every other person who has been killed in this country.”

Ms Bottoms was flanked by Dr King’s daughter, Bernice King, and the rappers TI and Killer Mike.

Rapper Killer Mike made an emotional plea to protesters to think about their actions and not destroy the communities which they live in.

He wept as he told protesters: “I woke up wanting to see the world burn down yesterday because I am tired of seeing black men die. He casually put his knee on a human being’s neck for nine minutes as he died like a zebra in the clutch of a lion’s jaw…”

He added: "We have to be better than this moment. We have to be better than burning down our own homes, because if we lose Atlanta, what else we got?"

Former police officer Derek Chauvin, 44, was charged with third-degree murder and second-degree manslaughter on Friday after the death of Mr Floyd.

Protests have continued following the death of George Floyd. Credit: Julio Cortez/AP

In Minneapolis, thousands of protesters marched through the city centre as an 8pm curfew ticked past.

“Prosecute the police!” some chanted, and “Say his name: George Floyd!”

There was no violence, but some protesters sprayed graffiti on buildings while elsewhere in the city, officers fired tear gas and rubber bullets to drive back crowds of protesters.

It was not clear if – or how – authorities would enforce the curfew amid questions about how city and state leaders have handled the crisis.

A protesters shouts at police during a protest in Los Angeles. Credit: AP

Chauvin, who was fired along with three other officers who were at the scene, faces more than 12 years in prison if convicted of murder.

A lawyer for Mr Floyd’s family welcomed the arrest but said he expected a more serious murder charge and wants the other officers arrested too.

Prosecutor Mike Freeman said more charges were possible, but authorities “felt it appropriate to focus on the most dangerous perpetrator”.

Minneapolis Mayor Jacob Frey said a curfew also will be in place on Saturday, and nearby cities announced similar restrictions.

“I know that whatever hope you feel today is tempered with scepticism and a righteous outrage,” Mr Frey said in a statement.

“Today’s decision from the County Attorney is an essential first step on a longer road toward justice and healing our city.”

A police car burns after protesters marched to the Georgia State Capitol Credit: Alyssa Pointer/AP

Protests nationwide are being fuelled by outrage over Mr Floyd’s death and years of police violence against African Americans.

Demonstrators clashed with officers in New York and blocked traffic in Houston and San Jose, California.

News of the arrest came moments after Minnesota governor Tim Walz acknowledged the “abject failure” in responding to this week’s protests and called for swift justice for the officers.

Demonstrators walk along Pennsylvania Avenue as they protest the death of George Floyd Credit: Evan Vucci/AP

Meanwhile, US president Donald Trump threatened action, tweeting “when the looting starts, the shooting starts”, which prompted a warning from Twitter for “glorifying violence”.

Mr Trump later said he was referring to shooting that had happened during the protests and said he had spoken to Mr Floyd’s family and “expressed my sorrow”.

The president called video of the arrest “just a horrible thing to witness and to watch. It certainly looked like there was no excuse for it”.