As events marking the anniversary of the shooting death of Michael Brown were winding down Sunday night, gunfire erupted in Ferguson, Mo., leaving a gunman in the hospital. Police say the suspect was not part of the weekend’s rallies, which have been peaceful.

The shooting, in which dozens of rounds were reportedly fired, began near the intersection of Ferguson Avenue and West Florissant Avenue, the epicenter of last summer’s standoffs between protesters and police.

Police say that the initial outburst of gunfire seemed to be between two groups, with as many as six people firing. Photos from the scene show two unmarked police cars with bullet holes.

St. Louis County Police Chief Jon Belmar said in a news conference that plainclothes officers pursued one of the suspects, who then fired upon them. Other officers were on the street, wearing riot gear.

The plainclothes officers returned fire, striking the suspect, Belmar said.

Belmar said the suspect is in surgery in “critical … unstable condition.”

The four officers have been placed on administrative leave.

The shooting began around 11:10 p.m. local time, says St. Louis Public Radio. The station reports, “Protesters and police both ran toward Ferguson Avenue, away from West Florissant, after the shots rang out, leaving the streets mostly clear by 11:30.”

Demonstrator Roberta Lynch, who held a cane as she ran from the shooting, told St. Louis Public Radio, “We need to stop, too much has been going on, too many people getting murdered already. This is senseless.”

Belmar described the individuals involved in the incident as “criminals” and distinguished them from protesters who are calling for positive changes in the community.

The shooting disrupted a march that was one of several events this weekend to mark the anniversary of the death of Michael Brown, an 18-year-old black Ferguson resident who was unarmed when he was shot and killed by (now former) police officer Darren Wilson, who is white.

St. Louis Public Radio reports that another shooting — a drive-by attack — took place at a nearby apartment building several hours after the violence at West Florissant Avenue. That shooting left two young men injured.

— via NPR’s The Two-Way