UPDATE: Prosecutors have filed 10 counts against Tyrone Harris, the 18-year-old man shot by police in Ferguson on Sunday night.
The St. Louis County Prosecutor announced charges that include five counts of armed criminal action, four counts of first-degree assault on a law enforcement officer and one firearms charge.
Mr Harris' father, Tyrone Harris Sr., says two girls who were with his son told him he was unarmed and "running for his life" when he was shot.
One year after 18-year-old Michael Brown was shot by a police officer in Ferguson, Missouri, his father has paid tribute to his son in moving scenes, saying "Nothing has changed for me. Nothing."
And his words proved tragically correct, as a memorial service and peaceful solidarity march to mark Brown's death were followed by another 18-year-old black man being critically injured by police after he opened fire on them.
Police officers had been tracking the armed man during the protest marking the a year since death of Brown, who was unarmed when he was killed by while police officer.
Brown's death has become symbolic for tensions between African-Americans and the police in the US after it sparked rioting and the national "Black Lives Matter" movement across America.
Michael Brown Jr's father, Michael Brown Sr
The injured man has been named as 18-year-old Tyrone Harris Jr, and is in hospital in a "critical, unstable" condition according to police.
According to Police Chief Jon Belmar, the man approached the officers, who were in an unmarked police van, and opened fire.
The officers returned fire from inside the vehicle, then pursued the man on foot. When he shot at them again, all four officers fired back. The man was shot and fell to the ground.
None of the officers was seriously injured. All four have been put on standard administrative leave. They were not wearing body cameras, Belmar said.
Speaking ahead of the anniversary, Michael Brown Sr said that even a year after losing his son he "hurts every day".
He said: "We never had a time to just sit and grieve so this just happened for me." While he acknowledged the Ferguson police force had made changes since his son's death, he said these were "too late for him, though they might help other families".
The chief said an estimated six shooters unleashed a "remarkable" amount of gunfire over about 45 seconds.
The shooting happened shortly after what the chief called "an exchange of gunfire between two groups" rang out around 11:15 pm on Sunday while protesters were gathered on West Florissant Avenue, which saw rioting and looting last year after Brown's killing.
The shots sent protesters and reporters running for cover.
Belmar said firmly that the people firing were not part of the Brown protest. "They were criminals. They weren't protesters," he said.
The man who fired on officers had a semi-automatic 9MM gun that was stolen last year from Cape Girardeau, Missouri, according to the police chief.
"There is a small group of people out there that are intent on making sure that peace doesn't prevail," he said. "There are a lot of emotions. I get it. But we can't sustain this as we move forward."
The anniversary of Brown's killing, which cast greater scrutiny on how police interact with black communities, has sparked days of renewed protests, though until Sunday they had been peaceful and without any arrests.
Before the gunfire, protesters were blocking traffic and confronting police. One person threw a glass bottle at officers but missed.
For the first time in three consecutive nights of demonstrations, some officers were dressed in riot gear, including bullet-proof vests and helmets with shields. Police at one point early Monday shot smoke to disperse the crowd that lingered on West Florissant, Belmar said.
One officer was treated for cuts related to a rock thrown at his face, and two officers were pepper sprayed by protesters, county police spokesman Officer Shawn McGuire said in an email. Five people were arrested, according to records McGuire released.
There were also reports of two separate shootings on nearby Canfield Drive, the street where Brown was shot, Belmar said. He had no further details about the incidents or whether anyone was injured.
Several other peaceful events earlier Sunday were held to mark the anniversary.
Brown's father, Michael Brown Sr, led a march through town. It started at the site where Brown was fatally shot by officer Darren Wilson. A grand jury and the U.S. Department of Justice declined to prosecute Wilson, who resigned in November.
Later, a few hundred people turned out at Greater St. Mark Family Church for a service to remember Brown, with his father joining other relatives sitting behind the pulpit.
Organisers of some of the weekend activities pledged a day of civil disobedience on Monday, but have not offered specific details.