A man who opened fire on officers in Ferguson, Missouri, is in "critical, unstable" condition after being struck when the officers returned fire, the St. Louis County Police chief said early Monday.

Chief Jon Belmar said at a news conference that plainclothes officers had been tracking the man, who they believed was armed, during a protest marking the anniversary of the death of unarmed black 18-year-old Michael Brown, whose shooting death by a white Ferguson police officer touched off a national "Black Lives Matter" movement.

The man approached the officers, who were in an unmarked police van, and opened fire, Belmar said. The officers returned fire from inside the vehicle and then pursued the man on foot when he ran.

The man again fired on the officers, the chief said, and all four officers fired back. He was struck and fell.

The man was taken to a hospital, where Belmar said he was in "critical, unstable" condition in surgery. He was not immediately identified.

None of the officers was seriously injured. All four have been put on standard administrative leave.

The shooting happened shortly after what the chief called "an exchange of gunfire between two groups" rang out around 11:15 p.m. Sunday while protesters were gathered on West Florissant Avenue, a business zone that saw rioting and looting last year after Brown's killing. The shots sent protesters and reporters running for cover.

The chief said an estimated six shooters unleashed a "remarkable" amount of gunfire over about 45 seconds.

Belmar waved off any notion that the people with the weapons were part of the 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 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 the 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 protests, some officers were dressed in riot gear, including bullet-proof vests and helmets with shields.

One officer was treated for cuts related to a brick thrown at his face, Belmar said. Police made an unknown number of arrests and at one point early Monday shot smoke to disperse the crowd that lingered on West Florissant, he said.

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.

Organizers of some of the weekend activities pledged a day of civil disobedience on Monday, but have not offered specific details.

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Associated Press reporter Jeff Roberson contributed to this report.