The US National Guard has been deployed in Minneapolis after violent clashes erupted following the death of an unarmed black man in police custody.
George Floyd, 46, died on Monday and video showed him gasping for breath as a white policeman knelt on his neck.
Protests are now taking place for a third night in the city.
Reports claimed one police station was overrun by demonstrators and officers were seen abandoning the building after it was set on fire. Gun fire sounds were also heard.
On Wednesday, police fired tear gas at demonstrators as businesses were looted.
There have also been protests in Chicago, Los Angeles and Memphis.
The killing added to longstanding anger over the police killings of black Americans, including the recent case of 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 of a night earlier had resulted in damage to many businesses.
“George Floyd’s death should lead to justice and systemic change, not more death and destruction,” he said in a statement calling on all protests to remain peaceful.
Minneapolis Mayor Jacob Frey called on Wednesday 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.