A young man shot by police on Sunday night is now in "unstable, critical" condition in hospital, after a day of peaceful protests to remember Michael Brown, the unarmed black 18-year-old who was shot dead by a white police officer in Ferguson, Missouri, one year ago.

The man was wounded after opening fire on officers in Ferguson on the one-year anniversary of Brown's death, St. Louis County's police chief said early on Monday.

Photos have emerged from the scene showing a young black man lying on the ground. Police say they shot at the man after tracking him and said that he was armed with a stolen gun.

None of the officers was seriously injured during the exchange of gunfire, police Chief Jon Belmar said at a news conference today. All four have now been put on standard administrative leave. They were not wearing body cameras at the time of the shooting.

The St. Louis County Police Department is responding to a shooting in the 9600 block of Glen Owen. Crimes Against Persons in route — St. Louis County PD (@stlcountypd)August 9, 2015

The man was taken to a hospital, where Belmar said he was in "critical, unstable" condition. Authorities didn't immediately release the identities of anyone involved, but Tyrone Harris told the St. Louis Post-Dispatch the injured man was his son, 18-year-old Tyrone Harris Jr.

The shooting happened shortly after what the chief called "an exchange of gunfire between two groups" rang out around 11.15pm on 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 march on Sunday was led by the late teen's father, Michael Brown Sr. At 12.02pm, the exact time that Brown was killed last year, Brown Sr. held four and a half minutes of silence to commemorate the four and a half hours that his son's dead body was left in the street. Two doves were released after the silence was broken.

Belmar told reporters he was angry that the shooting interrupted the weekend's events, which had otherwise transpired peacefully. "These are the exact kind of events we try to avoid," Belmar said. "I think it's unfortunate. We are trying to keep everybody as safe as we can."

Protests were also held in New York, where about 100 protesters in Brooklyn staged a "die-in" demonstration for four and a half minutes to mark Brown's death. A separate rally was also held Manhattan.

The Justice Department has called for a complete overhaul of the Ferguson Police Department after an investigation found that widespread constitutional violations and racist police tactics were commonplace in the city.

In the year since Brown's death, police officers have killed at least 1,083 Americans, according to data compiled by VICE News. So far, only 22 of the officers involved in those incidents have been indicted or charged.