A young man who was shot after opening fire on police in Ferguson, Missouri, is in a “critical, unstable” condition and undergoing surgery, police say.
Police had been tracking the individual, who they say was armed with a stolen gun, at a protest to mark the first anniversary of the killing of unarmed black teenager Michael Brown.
Four officers have been placed on administrative leave, Police Chief Jon Belmar said.
The shot man has not been identified.
Photos from the scene showed a young black man lying bloodied on the ground.
The shooting of 18-year-old Michael Brown by white police officer Darren Wilson in August 2014 sparked demonstrations across America.
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.
Mr 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 with flashing lights.
The suspect fired on the vehicle, Mr Belmar said, and the plainclothes detectives returned fire from inside the van.
The man ran behind a building and fired again at the officers, the police chief said, and all four fired back, wounding him.
The officers were not wearing body cameras, he added.
Mr Belmar described the incident as “a tragedy for the family of this man, and the officers involved.”
There was a “small group of people out there intent on making sure we don’t have peace that prevails,” he said.
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”.
“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.
The BBC’s Aleem Maqbool, who is in Ferguson, says there were not many people at Sunday’s gathering who felt anything had changed since Michael Brown was shot dead a year ago.
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