Shots were fired on Sunday during a march in Ferguson, Missouri, to mark the anniversary of the fatal shooting by a white police officer of unarmed black teenager Michael Brown, police said.
It was unclear who fired the shots or the extent of any injuries, a police spokesman told reporters, but initial reports suggested they were not aimed at the marchers.
About six shots were heard as 300 people made their way to a church on the outskirts of Ferguson as part of events to mark the death of Brown at the hands of a white police officer one year ago.
St. Louis County Police Chief Jon Belmar said he was angry that a shooting had marred the weekend's events in Ferguson, which have so far gone off without major incidents or arrests.
"These are the exact kind of events we try to avoid. I think it's unfortunate. We are trying to keep everybody as safe as we can," Belmar told reporters.
The march toward the church was halted while police and ambulances rushed to an area behind the church where the shots were thought to have come from.
Brown's shooting on Aug. 9, 2014, touched off months of protests nationwide about the treatment of minorities by police. Several other unarmed black men have since died in cities including New York, Cincinnati, Baltimore, and Arlington, Texas.