Shots have been fired in Ferguson, Missouri, at a protest to mark the first anniversary of the killing of unarmed black teenager Michael Brown.
St Louis County police said an officer was “involved in a shooting after coming under heavy gunfire” and that “two unmarked cars took shots”.
Eyewitnesses reported seeing at least one injured person lying on the ground.
The shooting of the 18-year-old by white police officer Darren Wilson sparked demonstrations across America.
It fuelled a national protest movement against racial bias by police.