FERGUSON, Missouri (CNN) -- A day of peaceful vigils to mark the one-year anniversary of Michael Brown's shooting death turned ugly late Sunday night when protesters threw rocks and bottles at officers, and police critically injured a man who they say fired at them.
The unidentified man in his 20s was undergoing surgery early Monday morning.
He unleashed a "remarkable amount of gunfire" against the officers using a stolen handgun, St. Louis County Police Chief Jon Belmar said.
"We cannot continue, we cannot talk about the good things that we have been talking about, if we are prevented from moving forward with this kind of violence," he said.
Belmar said those resorting to violence are not protesters.
"Protesters are people who are out there to effect change," he said. There were "several people shooting, several rounds shot."
Peaceful vigil, then chaos
The one-year anniversary observations of Brown's shooting death by a white Ferguson police officer started off peacefully Sunday.
Throughout the day, vigils honored the unarmed black teen. Attendants observed four and half minutes of silence, to signify the four and a half hours Brown's body lay on the street after he was shot last year.
But the suspect's gunfire shifted the focus Sunday night.
When officers first saw him, he was running away after allegedly exchanging gunfire with an unknown person.
Some gunfire rang out as reporters were talking to Ferguson's acting police chief, Andre Anderson. A startled Anderson continued speaking with a steady burst of gunfire in the background. Crowds scattered.
Detectives in an unmarked SUV turned on its emergency lights and pursued him, only to be shot at, according to Belmar. The bullets hit the vehicle's hood and windshield several times, Belmar said.
As the detectives got out of the car, the suspect allegedly turned around and fired again.
Then he ran toward a fenced area, where he continued firing -- until officers struck him multiple times, Belmar said.
The four plainclothes officers involved in the shooting have between six to 12 years of experience, he said. They have been placed on administrative leave.
Night turns tense
By then, police presence had turned heavy, and rumors about the shooting flew.
Police and protesters faced off in a tense standoff on West Florissant Avenue, where Brown was shot.
Several objects were thrown at police and some businesses damaged, the St. Louis County Police Department said. A journalist was attacked and robbed in a parking lot. Three St. Louis County police officers were injured: one was struck in the face by a brick while two others were pepper-sprayed.
Police, with helmets and shields, pushed crowds back and called in tactical units.
"We're ready for what? We're ready for war," some in the crowd chanted.
'Pray for peace'
Amid the chaos, some appealed for calm.
"Please pray for peace in Ferguson tonight and forever," Danny Takhar tweeted. "And the police department really needs to look at what they did last year and today."
Others posted a video of what they described as a victim of the shooting in Ferguson, lying on the streets bleeding.
"Please get him some help! He's bleeding out," a voice said off-camera.
The details of what happened on August 9, 2014, and the days of protest that followed have become a polarizing topic in Ferguson and America as a whole.
Brown's killing by officer Darren Wilson sparked outrage and protests nationwide against what some described as racial bias by the police.
A grand jury didn't indict Wilson and the Justice Department also declined to bring criminal charges.
The killing sparked weeks of protests that at times intensified into street fires and looting of businesses. Police fired tear gas in response, sparking more tensions.
Wilson retired from the Ferguson Police Department.
But protesters -- many of whom are skeptical of the local and federal inquiries into the case -- point to examples of police misconduct exposed in the wake of Brown's death. The case also led to new policing strategies, including police body cameras that have injected truth into areas where there was once only debate.
More protests planned
Early Monday morning, officers used smoke bombs to disperse the crowd.
But protesters are expected to return.
Events highlighting what's described as a day of civil disobedience are planned for Monday. In the past, such events have included blocking highways and shutting down big businesses across St. Louis County.
CNN's Sara Sidner reported from Ferguson, Missouri; CNN's Faith Karimi wrote from Atlanta. CNN's Joe Sutton contributed to this report