Protesters have clashed with police outside a "Black Lives Matter" conference in Cleveland, with images and video posted to social media showing an officer pepper-spraying the crowd.
The clashes began when police arrested a 14-year-old boy for allegedly having an open container of alcohol, reports Cleveland's ABC 5 news.
Witnesses say the teen was slammed to the ground during the arrest.
The boy had departed the conference at Cleveland State University and was among a group waiting for buses outside. Demonstrators say he was later released after they blocked the ambulance he was placed into.
The officer who dispensed pepper spray on the crowd was with the Cleveland Regional Transit Authority (RTA).
The RTA released a statement, saying the boy appeared to be too drunk to care for himself and the officers followed "normal procedure, which is after police collect pertinent information, juveniles are transported to police headquarters to await release to a parent or legal guardian".
"Within minutes, a large crowd had gathered," says the statement. "For the safety of the juvenile, Transit Police moved him from the open shelter area into a police cruiser.
"The crowd then surrounded the car and attempted to remove the juvenile from the car. By this time, several other law enforcement agencies had also responded.
"The crowd kept the police car from leaving the area. A Transit Police officer used a general burst of pepper spray in an attempt to push back the crowd, to no avail."
The conference kicked off in Cleveland on Friday (local time), with more than 1200 attendees pre-registered and more expected to have attended throughout the weekend.
Cleveland has been a focus of the Black Lives Matter campaign after the shooting death of 12-year-old Tamir Rice by a police officer and a 2012 shooting of two unarmed people following a police chase.