Prosecutors in the US town of Ferguson, Missouri, have charged a man with assaulting officers during events marking the first anniversary of the killing of unarmed black teenager Michael Brown.
They named the suspect as Tyrone Harris, 18, who was critically wounded in a gun battle with police on Sunday, Missouri time.
The authorities have declared a state of emergency in the city.
Michael Brown's death triggered a wave of demonstrations over the use of excessive force by police officers.
Mr Brown, 18, was shot in August 2014 by white police officer Darren Wilson, who was later cleared of any wrongdoing by a grand jury and the US Department of Justice.
Separately on Monday, about 50 people were arrested - including civil rights activist Cornel West - after blocking the main courthouse in St Louis.
The protesters were calling for the US government to disband the Ferguson Police Department, the St Louis Post-Dispatch reported.
Other protesters later briefly blocked a major highway - Interstate 70 - during the afternoon rush hour and a number of arrests were made.
The demonstrations were part of a day of civil disobedience called by activists in St Louis and other major US cities.
On Sunday, hundreds of people stood silent for four-and-a-half minutes at the spot where Mr Brown was killed in Ferguson, representing the number of hours that his body lay in the street unattended.
But the day of peaceful and sombre commemorations turned violent late at night when, according to police, a gunfight erupted between two rival groups on West Florissant Avenue.
Police say Mr Harris was armed and opened fire, but his father has described their account of events as "a bunch of lies".
Prosecutors have filed 10 charges against Mr Harris, including assaulting an officer.
Police said he was among six people who fired at an unmarked SUV with plainclothes detectives inside, who then fired back.
But the man's father told AP that his son, a close friend of Mr Brown, was shot between eight and 12 times as he was running away.
Authorities urge end to violence
In a statement on Monday, Ferguson Mayor James Knowles III said the city was "deeply disappointed" by Sunday's violence and urged local people to be "watchful for those who want to cause harm to our community".
"This kind of behaviour is counterproductive, and cannot be tolerated if we want to continue bettering the community for all our residents," he said.
Announcing a state of emergency on Monday, authorities in St Louis County placed police chief Jon Belmar in charge of Ferguson and the surrounding areas.
US attorney Loretta Lynch denounced Sunday's violence.
"As we have seen over the recent months and years, not only does violence obscure any message of peaceful protest, it places the community, as well as the officers who seek to protect it, in harm's way," she said.
Protest groups, including the Ferguson Action Council and Organization for Black Struggle, described Sunday's police response as "excessive".