Protests broke out on Monday night in the western part of the US city of Philadelphia after police shot and killed a black man they said had been armed with a knife.
The violence, which injured several police officers, is the latest in months of anti-racism protests across the US since the death in May of African American George Floyd, 46, after a Minneapolis police officer knelt on his neck for almost nine minutes.
In the early hours of Tuesday, a pickup truck drove through the protest area, hitting a female police officer. In video purporting to be of the incident shared on social media, a bystander said: "Oh my god. He hit a cop."
The 56-year-old officer was taken to hospital and is being treated for a broken leg, said Philadelphia Police Sergeant Eric Gripp. The empty truck was later found nearby.
Four other officers were hit with bricks when Monday's protest outside a police station in Philadelphia turned violent and they had to be taken to hospital, broadcaster NBC said.
Earlier, a man identified by officials as Walter Wallace, 27, had approached two police officers who drew their guns after warning him to put down the knife, video of the afternoon's shooting on social media showed.
The incident raised questions and was being investigated, Mayor Jim Kenny and Police Commissioner Danielle Outlaw said separately.
"The Officer Involved Shooting Investigation Unit of the Philadelphia police department will conduct a full investigation," Mr Kenney said.
Ms Outlaw said she heard "the anger of the community" after Wallace's death.
An unverified video on social media, apparently recorded by a bystander, showed the two officers point their guns at Wallace as he walked in the street.
He approached them as they backed away, with guns still aimed at him, while yelling to him to put down his knife.
Both then fired several shots and Wallace collapsed in the street, according to the video.
The Pennsylvania chapter of the American Civil Liberties Union called for transparency in the investigation into the shooting and a step-change in police and community provision of mental health services.
“This city is overdue for a reckoning with the brazenly violent and abusive behaviour in its police department," said Reggie Shuford, executive director of the ACLU of Pennsylvania.
"In 25-50 per cent of cases in which a person is killed by a police officer, the victim was in the midst of a mental health crisis. There is a better way to handle these incidents," he said.
Tension rose as the death of Wallace sparked clashes between protesters and police.
Since the incident in Minneapolis, Americans have staged nationwide demonstrations demanding racial equality and an end to police brutality, with protests sometimes turning violent.
