A police station in Minneapolis has been set alight during a third night of protests over the death of an unarmed black man in custody on Monday.
A police officer was filmed kneeling on the neck of George Floyd, 46, despite him saying he could not breathe.
President Donald Trump said “thugs” were dishonouring his memory and called on the National Guard to restore order.
The incident has added to anger over police killings of black Americans, including Breonna Taylor in Kentucky.
Mr Floyd’s family have demanded that the four police officers implicated in his death face murder charges. Prosecutors have said they are still gathering evidence.
A CNN journalist, Omar Jimenez, and his camera crew were arrested live on air by Minnesota state police officers on Friday morning, apparently because they did not move on when instructed.
The team was released an hour later, after the governor apologised for the arrest.
There have also been demonstrations in other US cities, including New York, Los Angeles, Chicago, Denver, Phoenix and Memphis.
Twitter accused Mr Trump of glorifying violence in a post that said: “When the looting starts, the shooting starts.”
What is the latest in Minneapolis?
On Thursday, protesters gathered outside the police department’s 3rd Precinct, the epicentre of the unrest.
Officers fired tear gas and rubber bullets in an attempt to disperse the crowd. But the cordon around the police station, which is near where Mr Floyd died, was breached by protesters, who set fire to it and two other nearby buildings as the officers withdrew.
Minneapolis Mayor Jacob Frey said there had been no choice but to evacuate the police station, adding: “The symbolism of a building cannot outweigh the importance of life, of our officers or the public.”
He called the unrest “unacceptable”, but recognised there was “a lot of pain and anger”.
He spoke after a tweet from Mr Trump blamed Thursday’s violence on a “lack of leadership” in Minneapolis and warned that he would send in the National Guard and “get the job done right” if Mr Frey failed to restore order.
National Guard personnel are normally under state control, although they can be put under federal control in emergencies.
Mr Trump also tweeted: “Any difficulty and we will assume control but, when the looting starts, the shooting starts.”
On Friday, the White House Twitter account quoted the president’s tweet on looting and shooting.
Twitter placed a “public interest notice” on the messages, saying they “violate our policies regarding the glorification of violence based on the historical context of the last line, its connection to violence, and the risk it could inspire similar actions”.
The historical context arises from riots in Miami in 1968. A federal task force found that the use of the phrase about looting by the Miami police chief at the time was a prime factor in the discontent that triggered the unrest.
Earlier on Thursday, Minnesota Governor Tim Walz had activated the state’s National Guard troops at the request of the mayors of Minneapolis and the neighbouring city of St Paul, declaring the situation a “peacetime emergency”.
Calling for peaceful protests, “not more death and destruction”, Mr Walz said the looting, vandalism and arson of Wednesday night had damaged many businesses, including ones owned by minorities.
What has been the reaction?
The four policemen involved have been fired, and the mayor has called for criminal charges against the officer seen pinning Mr Floyd.
Speaking through tears, Mr Floyd’s brother, Philonise, told CNN on Thursday the officers who “executed my brother in broad daylight” must be arrested and that he was “tired of seeing black men die”.