St. Louis County declared a state of emergency Monday as authorities arrested dozens of demonstrators here and charged an 18-year-old black man who had been critically wounded by police with assaulting law enforcement officers.

One year after Michael Brown died by police gunfire and violent clashes shook this suburb of St. Louis, Ferguson was again rocked by fear, anger and the threat of growing violence. By late Monday, police, protesters and residents were bracing for another night of tense demonstrations and uncertainty.

Some of the unrest was related to the anniversary of Brown’s death, including the arrest of nearly 60 people during a “day of resistance” Monday in St. Louis. Police jailed some of the most prominent leaders of the Black Lives Matter movement — DeRay Mckesson, Johnetta Elzie and Kayla Reed — as well as Princeton University professor Cornel West.

“Between the continuous death of unarmed teenagers at the hands of police [and] tear gassing on Canfield Drive . . . we see that not enough has changed in Ferguson,” said Brittany Packnett, an activist who works closely with Mckesson, Elzie and Reed. “It is August 2014 all over again.”

The largely peaceful protests began Sunday morning with a silent march, but by night they had been overtaken by what appeared to be random violence and opportunistic looting. The man shot by police, Tyrone Harris Jr., had been with two friends who were trying to sell a looted flat-screen television, according to his girlfriend and relatives. They said an argument with a buyer over the TV’s price escalated into a gunfight, during which police said Harris opened fire on a carload of undercover officers.

Harris, 18, remained in critical condition Monday. Police charged him with 10 counts of assaulting law enforcement, shooting at a motor vehicle and armed criminal action.

Harris’s shooting Sunday near midnight followed a mostly calm day of protests and vigils commemorating the anniversary of Brown’s death. Brown, an unarmed 18-year-old black man, was shot and killed by Darren Wilson, a white Ferguson police officer.

Brown’s killing last year thrust this small city into the center of a national conversation about police use of deadly force against minorities, even as it sparked violent unrest in Ferguson and beyond. It was unclear Monday what the fallout would be from the shooting of Harris — who attended the same school as Brown and knew him personally, according to school officials — but authorities throughout the region were preparing for more violence.

By Monday evening, activists had briefly shut down a major freeway before police cleared them out, and an uneasy lull was settling over the city as police and protesters awaited nightfall.

Ferguson has experienced this chaotic mixture of gun violence, looting and otherwise peaceful protests several times over the past year. With each incident, distrust between police and many in the community has deepened.

Just hours after police shot Harris, another burst of gunfire erupted near the Canfield Green apartment complex, close to where Brown was shot a year ago. Two young men — 17 and 19 years old — were wounded.

The two teenagers told police they were walking on the sidewalk when a man wearing a red hooded sweatshirt started shooting at them from the rear passenger side of a vehicle. The 17-year-old was shot once in the chest or shoulder, while the 19-year-old was shot once in the chest, police said. They were both taken to a hospital with what police described as non-life-threatening injuries.

Meanwhile, Paul Hampel, a reporter for the St. Louis Post-Dispatch, was beaten and robbed while reporting on a break-in happening on West Florissant Avenue.

“I got swarmed, beaten down really bad,” he told the Post-Dispatch.

St. Louis Alderman Antonio French, who was just a few feet away when the night’s first round of gunfire broke out, said he ran toward the office of a nonprofit group he founded called Heal STL. There he saw a young man who looked equally horrified by the shooting, so French invited him in.

Moments later, as French turned his back, the boy dashed out the door with six of the nonprofit’s iPads, a laptop and a camera bag.

“For a small group of people, the anniversary of Michael Brown’s death might as well have been Looting Day,” French said.

Police said three officers were injured Sunday night and Monday morning. Two officers were pepper-sprayed and one was hit in the face by a rock, a spokesman said.

“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 declaring a state of emergency. “The time and investment in Ferguson and Dellwood will not be destroyed by a few that wish to violate the rights of others.”

On Monday, St. Louis County Police Chief Jon Belmar took command of police in Ferguson. At an early-morning news conference, he described how Sunday’s events rapidly turned violent.

Police, he said, had responded to reports of looting on West Florissant, which has served as the focal point for the protests and unrest. Tensions rose around 8 p.m. as many demonstrators gathered for a rap concert and panel discussion.

After groups of young men began breaking into storefronts, police said, authorities cut off traffic to the area and deployed officers in riot gear. That response prompted angry locals and some protesters to gather in a face-off against the line of officers. Meanwhile, plainclothes detectives farther down the avenue were monitoring two rival groups of young men, including Harris, whom they believed to be armed.

The groups exchanged between 40 and 50 shots in about 45 seconds, “a remarkable amount of gunfire,” Belmar said.

Belmar said Harris then broke away from the rival groups. As an unmarked police SUV with flashing interior lights zoomed toward him, police said, Harris began firing at the vehicle, striking the hood and windshield multiple times.

The detectives returned fire. Harris was later found to be armed with a 9mm Sig Sauer pistol that had been reported stolen last year, Belmar said.

“They were criminals,” Belmar said of the two groups shooting. “They weren’t protesters.”

Harris was shot in the arm, legs, back, chest, liver and groin and taken to Barnes-Jewish Hospital, said his father, Tyrone Harris Sr., adding that his family has been denied entry. Relatives said they do not believe Harris was armed, though they acknowledged that he faced felony charges last year that involved a high-speed chase in a stolen vehicle. He was charged in November with stealing a car, stealing a firearm and resisting arrest, according to court records.

Harris’s girlfriend, Qunesha Coley, who was present Sunday night when the shooting erupted, said she met her boyfriend on West Florissant earlier in the evening.

As hundreds were on the street chanting “No justice, no peace” and “Hands up, don’t shoot,” Harris was with two friends who were carrying a looted flat-screen TV and were trying to sell it to another man, she said.

Harris’s friends wanted $40 but the buyer offered $20, other witnesses said, according to two relatives in interviews at Harris’s house. Coley said the buyer may have been trying to steal the TV when the shooting began.

Coley said Harris pushed her out of the way, saying he was trying to protect her, then took off running toward her red Chevrolet Impala. The plainclothes police, it turned out, were stationed right near her car, she said.

“I don’t understand why they would gun him down if he’s running towards the car,” Coley said. “Y’all gunned him down because he’s running towards y’all? And then had the nerve to put him in handcuffs after y’all gunned him down?”

The four detectives involved in the shooting were placed on administrative leave; they have not been identified. None were wearing body cameras — something activists have demanded in the past year.

Belmar, during his news conference early Monday, stressed that the groups shooting at each other were separate from the protesters demonstrating in the streets.

“There is a small group of people out there who are intent on making sure we don’t have peace that prevails,” Belmar said. He characterized the shooting as “avoidable.”

Harris’s father, however, said he has deep doubts in the police’s account of the shooting. “There ain’t no child perfect. And I don’t claim he’s a perfect person,” he said. “He had gotten in trouble with law and made some mistakes, but he was trying to get his life together.”

Harris’s girlfriend said Harris was not participating in the looting carried on by his two friends.

But after all that happened — the looting, the shootout with the buyer-turned-robber, and the police opening fire on her boyfriend — in the end, she noted, the TV that both sides were fighting over ended up busted on the street.

Berman and Wan reported from Washington. Wesley Lowery in Ferguson and Julie Tate, Jennifer Jenkins, Alice Crites, J. Freedom du Lac, Justin Wm. Moyer, Abigail Ohlheiser and Nick Kirkpatrick in Washington contributed to this report.