The troubled community of Ferguson is again on edge after a protest marking the anniversary of Michael Brown's death was punctuated with gunshots.
Police also critically wounded a black teenager accused of opening fire on officers.
Police, protesters and people who live and work in the St Louis, Missouri suburb were braced for what nightfall might bring following more violence along West Florissant Avenue, the same thoroughfare that was the site of massive protests and rioting after Mr Brown was fatally shot last year in a confrontation with a Ferguson officer.
"Of course I'm worried," said Sandy Sansevere, a retired health care worker who volunteers at the retail store operated by the non-profit group I Love Ferguson, which was formed after Mr Brown's death, to promote the community.
"What scares me are the guns."
Mr Brown, 18, was a young black man who was killed by a white Ferguson police officer, sparking a national debate over police treatment of minorities and the Black Lives Matter movement.
Several hundred people had gathered along West Florissant by 9pm, chanting and holding signs.
Overnight, police made several arrests after protesters blocked a traffic lane. Officers with bullhorns directed protesters to clear the road, and others in riot gear forced people out of the street.
Some demonstrators threw water bottles and other debris at officers.
St Louis County police chief Jon Belmar said: "They're not going to take the street tonight. That's not going to happen."
Several people were handcuffed and put into vans. "What did I do?" one woman asked repeatedly.
As officers were clearing the road, one officer fired pepper spray into a crowd of people, hitting a video journalist.
On Monday, the father of Tyrone Harris, the 18-year-old suspect who was shot called the police version of events "a bunch of lies".
He said two girls who were with his son told him he was unarmed and had been drawn into a dispute involving two groups of young people.
St Louis County Executive Steve Stenger declared a state of emergency, allowing Mr Belmar to take control of police emergency management in and around Ferguson.
Protests spilled outside of Ferguson. Almost 60 protesters were arrested at around midday for blocking the entrance to the federal courthouse in central St Louis. Authorities planned to release them on a promise to appear later in court.
Protesters later briefly blocked Interstate 70 during the rush hour, with more arrests made. Among those arrested at the court was scholar and civil rights activist Cornel West.
That protest, like other commemoration events over the past few days, was largely peaceful and sombre.
But on Sunday, several hundred people gathered in the street on West Florissant, ignoring an officer on a bullhorn repeatedly warning them to get to the pavement or face arrest.
Eventually, a few lobbed glass bottles and rocks at officers. One officer was taken to hospital with facial cuts after being hit with a rock.
Two others had minor injuries after protesters pepper-sprayed.
As tensions escalated, several gunshots suddenly rang out from the area near a strip of stores, including some that had been looted moments earlier. Mr Belmar believes they came from about six different shooters.
What prompted the shooting was not clear, but Mr Belmar said two groups had been feuding. The shots sent protesters and reporters running for cover.
The shooters included Tyrone Harris, whom police had been watching out of concern that he was armed, Mr Belmar said.
During the gunfire, the suspect crossed the street and apparently spotted plainclothed officers arriving in an unmarked van with distinctive red and blue police lights, Mr Belmar said.
The suspect allegedly shot into the windscreen of the van.
The four officers in the van fired back, then pursued the suspect on foot. The suspect again fired on the officers when he became trapped in a fenced-in area, the police chief said, and all four opened fire.
Mr Harris is in a critical condition after undergoing surgery. Prosecutors announced 10 charges against him - five counts of armed criminal action, four counts of first-degree assault on a law enforcement officer and a firearms charge. All 10 are felonies.
All four officers in the van, each wearing protective vests, escaped injury. They were not wearing body cameras, Mr Belmar said.
Tyrone Harris senior said his son was a close friend of Michael Brown and was in Ferguson on Sunday night to pay respects.
He said his son got caught up in a dispute among two groups of young people and was "running for his life" after gunfire broke out.
"My son was running to the police to ask for help, and he was shot," he said. "It's all a bunch of lies. They're making my son look like a criminal."
Mr Belmar said the suspect who fired on officers had a semi-automatic 9mm gun stolen last year from Cape Girardeau, Missouri.
Mr Belmar drew a distinction between the shooters and the protesters. "They were criminals," he said of those involved in gunfire. "They weren't protesters."
Missouri governor Jay Nixon agreed, saying such "reprehensible acts must not be allowed to silence the voices of peace and progress".
But some protest groups said police were too quick to go into riot mode and others questioned why plainclothed officers were part of the patrol.