FERGUSON, Mo. » 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 18-year-old Michael Brown.

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.

The shooting happened shortly after what the chief called "an exchange of gunfire between two groups" rang out around 11:15 p.m. Sunday as protesters were gathered on West Florissant Avenue, sending the crowd running for cover.

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.

Several other events earlier Sunday marked the anniversary of the killing, which cast greater scrutiny on how police interact with black communities. Brown’s father, Michael Brown Sr., led a march through town after a crowd of hundreds observed 4½ minutes of silence.

The group began their silence at 12:02 p.m., the time Brown was killed, for a length of time that symbolized the 4½ hours that his body lay in the street after he was killed. Two doves were released at the end.

The march started at the site where Brown, who was black and unarmed, was fatally shot by Ferguson officer Darren Wilson on Aug. 9, 2014. A grand jury and the U.S. Department of Justice declined to prosecute Wilson, who resigned in November, but the shooting touched off a national "Black Lives Matter" movement.

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.

The anniversary has sparked days of renewed protests, though until Sunday they had been peaceful and without any arrests.

Before the shots were fired, 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.

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

Earlier, at the march, some wore T-shirts with likenesses of Brown or messages such as, "Please stop killing us," or, "Hands up! Don’t shoot!" which became a rallying cry during the sometimes-violent protests that followed the shooting a year ago.

But the focus of the weekend was largely on Brown, who graduated from high school weeks before the shooting and planned to go to trade school to study to become a heating and air conditioning technician.

Relatives and friends described Brown as a quiet, gentle giant who stood around 6-foot-3, weighed nearly 300 pounds and was eager to start technical college. But police said Brown stole items from a convenience store and shoved the owner who tried to stop him on the morning of the shooting. Moments later, he and a friend were walking on Canfield Drive when Wilson, who is white, told them to move to the sidewalk.

That led to a confrontation inside Wilson’s police car. It spilled outside, and Wilson claimed that Brown came at him, menacingly, leading to the fatal shooting. Some witnesses claimed Brown had his hands up in surrender. Federal officials concluded there was no evidence to disprove testimony by Wilson that he feared for his safety, nor was there reliable evidence that Brown had his hands up in surrender when he was shot.

The shooting led to protests, some violent, and the unrest escalated again in November when a St. Louis County grand jury determined that Wilson did nothing wrong. He resigned days later. The November riots included fires that burned more than a dozen businesses.

The Justice Department reached the same conclusion in March, clearing Wilson. But in a separate report, the Justice Department cited racial bias and profiling in policing as well as a profit-driven municipal court system that often targeted black residents, who make up about two-thirds of Ferguson’s populace.

Ferguson’s city manager, police chief and municipal judge resigned within days of that report. All three were white. The new judge, interim city manager and interim police chief are all black.

Associated Press reporter Jeff Roberson contributed to this report.