The city of Ferguson has declared state of emergency amid protests marking the first anniversary of the death of black teenager Michael Brown.
Ferguson is reported to be tense after another black teenager, Tyrone Harris, was critically wounded in a gun battle with police on August 9.
Tyrone Harris, 18, was later charged with assaulting police officers.
Missouri Governor Jay Nixon has appealed for protests to remain peaceful.
Michael Brown was shot in August 2014 by white police officer Darren Wilson, who was later cleared of any wrongdoing by a grand jury and the DoJ.
The 18-year-old’s death triggered a wave of protests over alleged racism and the use of excessive force by police officers.
On August 10, police arrested several protesters who blocked a traffic lane on West Florissant Avenue – the site of previous unrest. Witnesses said some people in the crowd threw water bottles at officers but there were no reports of violence.
Police also arrested about 50 protesters – including civil rights activist Cornel West – who staged a sit-in outside the main courthouse in St Louis.
Other protesters also 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.
The state of emergency was issued on Monday afternoon by St Louis County Executive Steve Stenger and authorized county Police Chief Jon Belmar to take control in and around Ferguson.
Sunday’s shooting came after a day of peaceful and somber commemorations in memory of Michael Brown.
According to police, a gunfight erupted between two rival groups in West Florissant Avenue and a suspect, whom they identified as Tyrone Harris, then shot at plain-clothes police who returned fire.
Police Chief Belmar said those who had opened fire on the officers “were criminals, they weren’t protesters”.
However, Tyrone Harris’s father described the police account as “a bunch of lies”, saying his son had been unarmed and was “running for his life”.
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.
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 vandalizing cars in Arlington, Texas, on August 7.