The St. Louis County police said frozen water bottles were thrown at officers, prompting them to order the crowd to disperse or face arrest.
"Safety, our top priority, is now compromised. This is no longer a peaceful protest. Participants are now unlawfully assembled," the department tweeted
"The recent acts of violence will not be tolerated in a community that has worked so tirelessly over the last year to rebuild and become stronger," St. Louis County Executive Steve Stenger said in a statement.
The executive order put St. Louis County Police Chief Jon Belmar in charge of police operations in Ferguson and the surrounding areas, Stenger said.
During the day Monday, roughly 200 demonstrators marched from Christ Church Cathedral to the Thomas F. Eagleton United States Courthouse in St. Louis. The protesters carried signs, chanted and prayed and demanded the Justice Department take action.
At the Old Courthouse in downtown St. Louis, protesters hung a banner from two balloons. It read, "Racism still lives here #fightback."
Police arrested 56 people at the courthouse demonstration, St. Louis Metropolitan Police Department spokeswoman Schron Jackson told CNN.
Video posted to social media showed officers arresting several prominent protesters, including activist and intellectual Cornel West, who was also arrested during an October protest
at the Ferguson Police Department.
One of those protesters, Johnetta Elzie, who has been a mainstay of the demonstrations and goes by Netta, tweeted minutes before her arrest, "If I'm arrested today please know I'm not suicidal. I have plenty to live for. I did not resist, I'm just black."
Later Monday, another group of protesters blocked part of Interstate 70 in Earth City, Missouri. Some of them held yellow signs that said, "Ferguson is everywhere."
Protesters held hands and formed a line across the highway. About 20 minutes later, troopers cleared the roadway, walking with protesters toward the shoulder and apparently arresting some of them in a nearby parking lot.
Shooting suspect charged
Monday's acts of civil disobedience came after a night of violence that left Ferguson on edge.
Peaceful marches in the St. Louis suburb planned by day on Sunday were shattered that same night when gunfire broke out, sending protesters and police scattering to safety.
The alleged gunman, 18-year-old Tyrone Harris of St. Louis, is hospitalized in critical condition and in police custody. The St. Louis County Police Department said officers shot the teenager after he unleashed a "remarkable amount of gunfire" at police -- a characterization the man's aunt contends is not true.
Prosecutors have charged Harris with four counts of first-degree assault on law enforcement, five counts of armed criminal action and one count of discharging a firearm at a motor vehicle, St. Louis County Police Department spokesman Sgt. Brian Schellman said.
Belmar said earlier that Harris used a stolen handgun to fire at officers.
Harris is being held on a $250,000 bond, Schellman said.
Harris' aunt, Karen Harris, said her nephew attended the protests because he was friends with Brown. Recounting what other family members who were with Tyrone Harris described, the aunt said Tyrone Harris wasn't carrying a gun and never fired at police.
He was "running for his life" just like everyone else, she said, when the gunshots were fired.
Peaceful vigil, then chaos
The anniversary observations of Brown's shooting death by a white Ferguson police officer started off peacefully Sunday.
Vigils honored him throughout the day. Attendees observed four and a half minutes of silence to signify the four and a half hours Brown's body lay on the street after the unarmed black teen was shot last year.
But the new gunfire shifted the focus Sunday night.
When officers first saw the suspect, he was running away after exchanging gunfire with an unknown person, police said.
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 the suspect, 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.
"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," Belmar said, adding that 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."
Night turns tense
By Sunday night, police presence had turned heavy, and rumors about the shooting flew.
Police and protesters faced off in a tense standoff on West Florissant Avenue, not far from Canfield Drive, 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.
In a separate incident early Monday, a man wearing a red hooded sweatshirt shot two teens, 17 and 19, in the chest, the St. Louis County Police Department said.
Both were hospitalized with injuries not considered life-threatening, authorities said.
The teens were walking on a sidewalk near where Brown was killed a year ago.
'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 shooting victim in Ferguson lying on the streets bleeding.
"Please get him some help! He's bleeding out," a voice said off camera.
Ferguson city officials condemned the violence in a statement Monday.
"This kind of behavior from those who want to cause disruption and destroy the progress from this past year will not be tolerated," the city's mayor and City Council said. "We are asking for our citizens and businesses to be diligent and to be watchful for those who want to cause harm to our community."
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 U.S. Justice Department also declined to bring criminal charges, but the feds did issue a report that found the Ferguson Police Department and the city's municipal court had engaged in a "pattern and practice" of discrimination against African-Americans, targeting them disproportionately for traffic stops, use of force and jail sentences.
Brown's 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 the introduction of police body cameras.