Shots have been fired in Ferguson, Missouri, at a protest to mark the first anniversary of the killing of unarmed black teenager Michael Brown.
Photos from the scene showed a young black man with heavy bloodstains lying on the ground.
St Louis County police said an officer was “involved in a shooting after coming under heavy gunfire” and that “two unmarked cars took shots”.
A police news conference is expected later.
The shooting of 18-year-old Michael Brown by white police officer Darren Wilson sparked demonstrations across America.
It fuelled a national protest movement against racial bias by police.
The shots were heard at around 23:15 on Sunday (04:15 Monday GMT) as a large crowd gathered on West Florissant Avenue, at the end of a sombre, peaceful day of commemorations.
In the early hours of the morning, police used loudspeakers to urge people to disperse, saying “this is no longer a peaceful protest”.
“We intend to use chemical munitions,” the announcement said.
Eyewitnesses on Twitter reported police using tear gas to disperse protesters.
Earlier on Sunday, hundreds of people stood silent for four and a half minutes at the spot where Michael Brown was killed, representing the number of hours that his body lay in the street, unattended.