Washington, Aug 10 (IANS): A young man was shot after he opened fire at the police in the US state of Missouri during protests marking the first death anniversary of Michael Brown, an unarmed 18-year-old black man who was fatally shot last August by a white police officer, the media reported on Monday.

St. Louis county Police Chief Jon Belmar said the man was in a critical, unstable condition and undergoing surgery at a hospital in Ferguson town, USA Today reported.

After a day of peaceful protests, gunfire erupted as tensions between police and protestors flared late Sunday night in Ferguson town's West Florissant Avenue, where Brown was shot dead by white police officer Darren Wilson on August 9, 2014.

Belamar said the shooting occurred after undercover detectives monitored an individual in the crowd who they believed was armed and had three or four possible armed acquaintances.

An earlier shootout broke out between two groups and the suspect ran away. He was then spotted by police detectives in an unmarked vehicle, which began driving towards them with the emergency lights activated. The police detectives fired at the vehicle, hitting it a number of times.

The police detectives then left the vehicle and exchanged gunfire with the man, and the suspect was shot multiple times, the police chief added.

At least two unmarked police cars were shot at, officials said.

Police, with helmets and shields, pushed crowds back and called in tactical units.

Throughout Sunday, peaceful vigils marked the anniversary of Brown's death. Attendees observed four-and-half minutes of silence -- the length of time Brown's body lay on the street after he was shot.