Large crowds clashed with police as rioters vandalized stores in downtown Chicago overnight Sunday, with officers returning fire after being targeted by gunfire.
More than 100 people were arrested and 13 officers wounded as rioters smashed windows and looted high-end stores, authorities said.
"Shots fired at police who returned fire," tweeted Chicago police spokesman Tom Ahern early on Monday, who indicated that no officers had been injured in that exchange.
Local media said law enforcement was unsure what had sparked the unrest, which came after a reported police shooting in the city's Englewood neighborhood.
Debris littered a parking lot outside a damaged Best Buy where people loaded merchandise into cars, while an Apple Store had its window smashed.
Protests over racism and police killings have erupted across the United States in recent months, ignited by the May 25 death in Minneapolis of unarmed African-American George Floyd at the hands of officers.
But Chicago Mayor Lori Lightfoot described the unrest as "abject, criminal behavior -- pure and simple" and "nothing to do with legitimate" protests.
"We have to also step up in a time of challenge like this one, and give no shelter to the criminals," she said, calling on residents to provide details of alleged rioters and shooters.
The Chicago Transit Authority suspended all train and bus services to the downtown area at the request of public safety officials. Bridges that lead into the city were also raised to limit access.