Two people were shot and more than 100 others arrested amid widespread looting and vandalism in downtown Chicago.
In the violence on Monday, a security guard and a civilian were struck by gunfire and taken to the hospital in critical conditions, reports Xinhua news agency.
A total of 13 police officers were also injured, including a sergeant who was struck by a bottle while another had his nose broken.
Five guns were recovered at the scenes.
At a press conference, Chicago Police Superintendent David Brown said the looting was releated to a shooting in Englewood on Sunday afternoon, when police officers responded to a report of a 20-year-old man with a gun.
The man fled and fired at officers, and the officers retaliated and struck the suspect, who was taken to the hospital and is expected to survive.
After the shooting, a crowd gathered in the area, Brown said.
The Chicago Police Department became aware of “several social media posts” about looting planned downtown, and reacted by deploying 400 officers to the downtown area.
The first looting incident happened in South Chicago, Brown said, but “soon, car caravans were headed into the Loop” to begin looting.
It is “pure criminality”, he said.
A video posted on social media showed a white car drove a store while people cheered on.
Other videosshowed crowds breaking windows and entering stores in business districts in downtown Chicago.
Chicago Mayor Lori Lightfoot called the act a “felony criminal conduct” and “an assault on our city”.
She called on the public to help apprehend the looters.
“To those who engaged in this criminal behaviour … we are coming for you. We are already at work in finding you and we intend to hold you accountable,” Lightfoot said at the press conference, adding: “There is no justification for criminal behavior ever.”
The police have started a neighbourhood protection plan; a team of detectives has been assigned to scour security footage of all incidents; access to downtown Chicago will be restricted from 8 p.m. to 6 a.m., effective Monday night; and all day-off has been cancelled for Chicago Police officers, local media reported.
For several hours on Monday morning, the bus and train service was suspended in downtown Chicago, and all downtown bridges except one were raised, according to a Chicago Transit Authority (CTA) announcement.
CTA restored normal service shortly before 8 a.m., but the major streets throughout downtown Chicago remain closed.