Curfews were imposed on major US cities as clashes over police brutality escalated across America with demonstrators ignoring warnings from President Trump that his government would stop the violent protests "cold."

Minneapolis, the epicenter of the unrest, was gripped by a fifth consecutive night of violence on Saturday with police in riot gear firing tear gas and stun grenades at protesters venting fury at the death of George Floyd, an unarmed black man, during an arrest in the city on Monday.

Los Angeles, Chicago and Atlanta were among two dozen cities ordering people to stay indoors overnight as more states called in National Guard soldiers to help control the civil unrest not seen in the United States for years.

From Seattle to New York, tens of thousands of protesters took to the streets demanding tougher murder charges and more arrests over the death of Floyd, who stopped breathing after Minneapolis police officer Derek Chauvin knelt on his neck for nearly nine minutes.

In Los Angeles, officers fired rubber bullets and swung batons during a testy standoff with demonstrators who set fire to a police car.

Police and protesters clashed in numerous cities including Chicago and New York, with officers responding to projectiles with pepper spray while shop windows were smashed in Philadelphia.

Trump blamed the extreme left for the violence, including widespread looting and arson in Minneapolis, saying rioters were dishonoring the memory of Floyd.