The US National Guard has been deployed to Minneapolis, Minnesota, following a double night of violent clashes between security personnel and protesters, angered by the death of an unarmed black man in police custody.
Forty-six-year-old George Floyd died on Monday and video showed him gasping for breath as a white policeman knelt on his neck.
Police fired tear gas at protesters as businesses were looted on Wednesday and Thursday.
There have also been demonstrations in Chicago, Illinois, Los Angeles, California, and Memphis, Tennessee.
The killing added to longstanding anger over the police killings of black Americans, including the recent cases of Ahmaud Arbery in Georgia and Breonna Taylor in Kentucky.
Minnesota Governor Tim Walz activated the state’s national guard troops on Thursday at the request of the mayors of Minneapolis and nearby St Paul, declaring the situation a “peacetime emergency”.
He said the looting, vandalism and arson had resulted in damage to many businesses, including ones owned by minorities.
On Wednesday, Minneapolis Mayor Jacob Frey called for criminal charges against the policeman who was filmed pinning down Mr. Floyd. Four police officers involved in the arrest have already been fired.
The incident echoes the case of Eric Garner, who was placed in a police chokehold in New York in 2014. His death became a rallying cry against police brutality and a catalyst in the Black Lives Matter movement.