The Minneapolis police officer who was seen on video kneeling on the neck of George Floyd, a handcuffed black man who died in custody after pleading that he could not breathe, was charged with murder on Friday.
Hennepin County Attorney Mike Freeman said Derek Chauvin was charged with third-degree murder and manslaughter, after the office gathered enough evidence to prove the case beyond a reasonable doubt.
Mr Freeman did not have immediate details, but said a criminal complaint would be made available later.
The charges come after three days of protests, which had been escalating in violence as demonstrators torched a police precinct that had been abandoned by officers.
The violent protests spread to several US cities on Thursday, with a police station set on fire, shops looted and vehicles being smashed.
The state of Minnesota sent reinforcements from the National Guard to Minneapolis and its sister city St Paul as people massed for a third day of protests.
Livestream video showed protesters entering a police station in Minneapolis not far from where Floyd, 46, was arrested on Monday, with fires breaking out soon afterwards.
Dozens of businesses across the Twin Cities boarded up their windows and doors to prevent looting and authorities in Minneapolis shut down rail and bus services until Sunday out of safety concerns.
In St Paul, clouds of smoke hung in the air as police armed with batons and wearing gas masks and body armor kept a watchful eye on protesters along one of the city’s main commercial corridors, where firefighters also sprayed water onto a series of small fires. At one point, officers stood in line in front of a Target, trying to keep out looters, who were also smashing windows of other businesses.
The violence prompted President Donald Trump to warn that he would intervene if Minneapolis Mayor Jacob Frey failed to bring the situation under control, accusing the city's administration of "a total lack of leadership".
In a second tweet, Mr Trump threatened to send in troops to Minneapolis, saying "when the looting starts, the shooting starts".
The tweet was later hidden by Twitter for "glorifying violence".
CNN reported that police arrested its reporter and crew covering the protests.
Omar Jimenez was handcuffed and taken into police custody during a live broadcast despite identifying himself to officers, CNN said, calling the arrests a violation of the news team's rights.
In Denver, Colorado protesters blocked traffic and smashed vehicles while running from gunfire and police tear gas.
Hundreds of demonstrators stood in the downtown streets and chanted as darkness fell outside the Colorado State Capitol, where protesters spray-painted graffiti and broke car windows. In other areas of downtown Denver, police fired gas canisters and used rubber bullets to drive away people who were blocking streets. The protest briefly spilled over onto Interstate 25, blocking all lanes of traffic until police used tear gas to drive them away.
The protests continued into the night, despite Denver Mayor Michael Hancock pleading for calm.
Several hundred people demonstrated in New York's Union Square on Thursday, leading to at least five arrests.
The death of Floyd was seen as the latest in a long series of unjustified police killings of black civilians.
In footage recorded by a bystander, Floyd can be seen pleading that he cannot breathe as a white officer kneels on his neck until he goes limp.
In a separate protest over alleged excessive use of force against black suspects, shots were fired during a demonstration in Louisville, Kentucky on Thursday night to demand justice for woman killed by police fire in a raid on her home.
Breonna Taylor, a 26-year-old emergency medical technician, was fatally shot eight times on March 13 after Louisville narcotics detectives knocked down her front door. No drugs were found in the home.
Louisville police said there were at least seven shooting victims, at least one of whom is in critical condition. A spokesman said all seven were civilians.
Floyd's arrest and subsequent death evoked memories of riots in Ferguson, Missouri in 2014 after a policeman shot dead an African American man suspected of robbery, and the case earlier the same year of New Yorker Eric Garner, who died after New York police put him in an illegal chokehold as they tried to detain him for selling cigarettes.
Earlier, the police officers in Floyd's arrest were sacked and local and federal investigators have said they were working the explosive case as fast as they could.
"The Department of Justice has made the investigation in this case a top priority," said Erica MacDonald, the US federal attorney for Minnesota.
"To be clear, President Trump, as well as Attorney General William Barr, are directly and actively monitoring the investigation in this case."
The White House said Mr Trump was "very upset" upon seeing the "egregious, appalling" video footage and demanded his staff see that the investigation was given top priority.
"He wants justice to be served," Mr Trump's press secretary Kayleigh McEnany told reporters.