Just hours after Loretta Lynch’s historic swearing in as the new U.S. attorney general and the first African-American woman to lead the Justice Department, mayhem erupted Monday in the streets of Baltimore following the funeral for Freddie Gray.

The 25-year-old Mr. Gray died of severe injuries on April 19, a week after being arrested, handcuffed and tossed into a police van. His spine was nearly severed and his larynx was crushed while in police custody, authorities have reported.

The city of 620,000 became the latest flashpoint in a national movement against law enforcement’s use of lethal force, which demonstrators say is disproportionately exercised against African-Americans and other minorities.

The deaths of Mr. Gray and black men in New York City, Ferguson, Mo., Cleveland, Charleston, S.C., and elsewhere in the nation at the hands of police have reignited a debate about race relations in the United States.

While several days of large, peaceful protests had taken place in Baltimore following Mr. Gray’s death, Monday’s action turned violent.

Scores of demonstrators — mostly teenagers and young adults — set fires, looted stores and threw rocks and bottles at police officers in parts of the city.

Police said at least 15 officers were injured — some with broken bones— and more than 200 people were arrested, including about 34 juveniles.

Television helicopters broadcast the eruption that spread through parts of West Baltimore near Mondawmin Mall. One band of about 50 youths stomped on the hood of a police cruiser and smashed in its windows. Another police cruiser was engulfed in flames. Another 140 cars owned by neighborhood residents also were burned or damaged.

Masses of looters broke into a CVS drugstore, eventually setting it ablaze. As smoke billowed into the afternoon air, a number of other businesses, including a liquor store and check cashing company, also were looted.

After Monday’s violence, Maryland Gov. Larry Hogan declared a state of emergency and activated the National Guard. Baltimore Mayor Stephanie Rawlings-Blake ordered a curfew from 10 p.m. to 5 a.m. that went into effect Tuesday.

On Tuesday, President Obama strongly condemned the riots, describing the looters as “thugs” who needed to be “treated as criminals.”

Ms. Lynch echoed his sentiments in a statement she released late Monday night.

“I condemn the senseless acts of violence by some individuals in Baltimore that have resulted in harm to law enforcement officers, destruction of property and a shattering of the peace in the city of Baltimore,” she said.

Many in Baltimore agreed, with volunteers and neighborhood residents, including youngsters, turning out Tuesday to sweep up glass and clear debris from damaged stores.

At a roundtable discussion on cybersecurity on Wednesday in Washington, Ms. Lynch said Baltimore could be seen as a symbol of the national debate on race relations and law enforcement.

“But, she added, “I’d ask that we remember that Baltimore is more than just a symbol. Baltimore is a city. It is a great city. It is a beautiful city.”

She said it is a city that police were trying to protect and peaceful protesters were trying to improve, all while “struggling to balance great expectations and need with limited resources.”

While Ms. Lynch has not yet traveled to Baltimore, she sent two officials from the department — Vanita Gupta, head of the federal Justice Department’s Civil Rights Division, and Ronald Davis, director of the Community Oriented Policing Services Office.

The Justice Department and the FBI have been investigating Mr. Gray’s death for possible civil rights violations. The Justice Department also is conducting a separate review of the Baltimore Police Department’s use of force practices. The police department requested the review, which is run by Mr. Davis’ office. Results are expected to be announced soon, Justice Department officials said.

The city has spent $6 million in the past four years to settle 100 cases of police brutality and other misconduct.

Additionally, Baltimore State’s Attorney Marilyn Mosby is expected to get a report from the city police department’s investigation on Friday, May 1. She then will decide whether to pursue charges against the six police officers that arrested Mr. Gray. Officials announced last week that all six have been suspended with pay.

Thousands of mourners attended Mr. Gray’s funeral Monday at New Shiloh Baptist Church in Baltimore. The 2,300-seat church was packed. Speakers included the Rev. Jesse Jackson, who said Mr. Gray was a victim of inequality. He said Mr. Gray’s poor West Side neighborhood needed investment, jobs and better housing.

“Why wasn’t Fred’s side of town developed?” Rev. Jackson asked. “Why can’t the west side get what downtown gets?”

He said there have been 110 deaths in Baltimore at the hands of police since 2010. “Fred wasn’t No. 1; he was number one-one-one,” Rev. Jackson said.

The curfew Tuesday was mostly obeyed. However, initially, about 200 protesters ignored warnings from police and pleas from pastors and other community activists to disperse. Some threw water bottles or laid down on the ground.

A line of officers behind riot shields later fired pepper balls at the crowd, which then dispersed in a matter of minutes.

Just before midnight Tuesday, Balti­more Police Commissioner Anthony Batts declared the curfew a success.

On Wednesday, city police and National Guard troops patrolled Baltimore’s streets, giving the city a semblance of calm. Peaceful demonstrations continued. Baltimore’s Major League Baseball team, the Orioles, played the Chicago White Sox at Camden Yards near the Baltimore Inner Harbor tourist area. But the game was played without any spectators at the stadium on the order of the Orioles management after consultation with MLB, state and local officials, a sign of the tenuous security situation.

Baltimore’s Symphony Orchestra staged an impromptu concert downtown as a demonstration of appreciation for the city.

Democratic presidential candidate Hillary Clinton also weighed in on the unrest in Baltimore during a speech Wednesday at Columbia University.

“The tragic death of another young African-American man. The injuries to police officers. The burning of peoples’ homes and small businesses,” she said. “We have to restore order and security. But then we have to take a hard look as to what we need to do to reform our system.”