US police have released video of a chase in Akron, Ohio, which ended in a black man being shot dead, hit more than 60 times by pursuing officers.
Police believe Jayland Walker, 25, opened fire first and officers feared for their lives during the night-time traffic stop on 27 June.
Mr Walker was not armed when he ran away from his car but police say a pistol was later found inside it.
Akron's mayor urged local people to show patience as protests began.
"The video is heart-breaking, it's hard to take in," Daniel Horrigan said after the footage was released on Sunday.
State Attorney General Dave Yost promised a "complete, fair and expert investigation" by the Ohio Bureau of Criminal Investigation while Akron police are holding a separate internal investigation into whether officers violated departmental rules or policies.
The eight officers involved in the shooting, seven of whom are white and one of whom is black, have been put on paid administrative leave.
A lawyer for Mr Walker's family said officers had kept firing even after he was on the ground.
Derrick Johnson, president of the National Association for the Advancement of Colored People, described Mr Walker's death as "murder. Point blank".
Jayland Walker's car was stopped on Monday 27 June at 00:30 for a routine traffic inspection.
A shot was heard from the car and a traffic camera captured what appeared to be a muzzle flash, Akron Police Chief Steve Mylett said, quoted by the Associated Press news agency.
Police body cameras show a person in a ski mask getting out of the car and running towards a car park, chased by officers for about 10 seconds before they open fire.
One officer tried first to use a stun gun but that was unsuccessful, police say.
Police Chief Mylett said one still photo seemed to show Mr Walker "going down to his waist area" and another appeared to show him turning toward an officer. A third picture image "captures a forward motion of his arm", he said.
As well as the pistol found with a loaded magazine on a seat in the car, a bullet casing matching the weapon was found nearby, police say.
Demonstrators marched peacefully through Akron and gathered in front of the city justice centre after the video was released.
Family lawyer Bobby DiCello said police had fired on Mr Walker even when he was on the ground and had handcuffed him before trying to provide first aid.
Mr Walker's family said in a statement to CBS News: "Anger is fine. Anger is understandable. Violence is not. Let's uphold the dignity of Jayland's life as we peacefully demand justice for him."