The US National Guard has been deployed to Minneapolis after two nights of violent clashes 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.
Police fired tear gas at protesters as businesses were looted on Wednesday. More protests are planned for Thursday.
There have also been protests 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 of a night earlier had resulted in damage to many businesses, including ones owned by minorities.
“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…
