St. Louis County Police say an officer who came under heavy gunfire returned shots amid a protest in Ferguson.

Chief John Belmar says two groups of people exchanged fire near where protests were taking place, but says the shooters were not protesters.

“No, they were criminals, they weren’t protesters,” Chief Belmar said at a news conference. “Protesters are people that are out there talking about a way to affect change, whatever that may be.”

Chief Belmar said 45 to 50 shots were fired in about 45 seconds, between the two groups. One officer was struck in the face with a brick and some stores were looted overnight. The police department tweeted that at least two unmarked cars were hit by gunfire.

Those incidents took place near where protests were taking place marking the one-year anniversary of the Michael Brown shooting.

Police shot one man that they say opened fire on an unmarked police SUV. The man, 18-year-old Tyrone Harris, was reported to be in critical and unstable condition early Monday morning.

For the first time in three nights of protests, some officers were dressed in riot gear, including bullet-proof vests and helmets with shields.

A grand jury and the U.S. Department of Justice declined to prosecute Ferguson Police officer Darren Wilson, who shot Brown, but the shooting touched off a national “Black Lives Matter” movement.

(AP and Missouri Net contributed to this report)