Prosecutors in 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.
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 police force by officers.
The 18-year-old 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.
What happened in Ferguson in 2014?
Separately on Monday, some 50 people were shown on local news networks being arrested after blocking the main courthouse in St Louis, including civil rights activist Cornel West.
Rallies were due to take place in parts of St Louis and other major US cities as part of a day of civil disobedience called by activists.
‘Bunch of lies’
Hundreds of people stood silent on Sunday for four-and-a-half minutes at the spot where Michael 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 fired, but his father has described their account of events as “a bunch of lies”.
Prosecutors filed 10 counts of charges against him, 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.
The gunmen “were criminals; they weren’t protesters,” he said, adding that the officers were not wearing body cameras – a major concern cited by protest groups. Four police officers have been placed on administrative leave.
The man’s father told the AP news agency that his son, a close friend of Brown, was shot between eight and 12 times as he was running away.
Photos from the scene showed a young black man lying bloodied on the ground. He was face down and handcuffed.
Announcing a state of emergency on Monday, the authorities in St Louis county placed police chief Jon Belmar in charge of Ferguson and the surrounding areas.
US attorney Loretta Lynch denounced the incident on Sunday, saying “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”.
It also drew criticism from protest groups, including the Ferguson Action Council and Organization for Black Struggle which described the police response as “excessive”.
John Gaskin III, a member of US rights group NAACP, told AP he did not expect unrest to escalate in Ferguson in the same way it has over the past year.
However, he warned, not many people would believe the police officers’ account of Sunday night’s events as “there’s still a tremendous level of distrust between law enforcement and the community”.
There have been a number of high-profile shootings of unarmed black men in US cities in recent months.
The most recent case involved 19-year-old Christian Taylor who was shot dead by a white police officer after he was spotted on surveillance footage vandalising cars in Arlington, Texas, on Friday.
The nationwide protest movement was further incensed when a justice department investigation found widespread racial bias in the Ferguson police force.
US Dept of Justice report on Ferguson racism