A man in Ferguson, Missouri, who was shot after police said he opened fire on them is in a “critical, unstable” condition and undergoing surgery.
He was at a protest to mark the first anniversary of the killing of unarmed black teenager Michael Brown.
Police said he was armed with a stolen gun and they had been tracking him. Four officers have been placed on administrative leave.
Photos from the scene showed a young black man lying bloodied on the ground.
The man’s name and age have not been released.
The shooting of 18-year-old Michael Brown by white police officer Darren Wilson in August 2014 sparked demonstrations across the United States.
It fuelled a national protest movement against racial bias by police.
What happened in Ferguson in 2014?
US Dept of Justice report on Ferguson racism
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.
St Louis County Police Chief Jon Belmar told reporters that after “an exchange of gunfire between two groups” the suspect, believed by police to be armed, left and encountered officers in an unmarked SUV.
The suspect fired on the vehicle, Mr Belmar said, and the plainclothes detectives returned fire from inside the van.
There was then a further exchange of fire after the officers left the vehicle.
The officers were not wearing body cameras, he added.
President Barack Obama announced a federal programme to help pay for lapel-mounted cameras for US police to record their interactions in the wake of the nationwide protests on perceived policing injustices.
In the early hours of the morning, police used loudspeakers to urge people still gathered at the scene to disperse, saying “this is no longer a peaceful protest”.
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.
The BBC’s Aleem Maqbool, who is in Ferguson, says most people at Sunday’s gathering felt little had changed since Michael Brown was shot dead a year ago.