BALTIMORE » Shortly before she became the youngest top prosecutor in any major American city, Marilyn J. Mosby, a daughter and granddaughter of police officers, had tough words about how the nation’s criminal justice system had handled mistreatment of black men by the police.

"It’s been 78 days since Michael Brown was shot in the street by a police officer," Mosby said at her alma mater, Tuskegee University in Alabama, in October. "It’s been 101 days since Eric Garner was choked to death in New York by a police officer, and 54 days since the New York City medical examiner ruled that incident a homicide. Neither has resulted in an indictment."

On Friday, Mosby made clear that she planned to proceed at a different pace. In stunning remarks that instantly transformed the mood here, she announced that she was charging six police officers with crimes including murder and manslaughter in the death of Freddie Gray, a 25-year-old black man whose fatal spinal cord injury in police custody had set off nearly two weeks of unrest, including a devastating night of looting and arson.

The dramatic announcement thrust Mosby, 35, an African-American, into the national spotlight, not only for the hard-hitting language she used in describing the behavior of the officers but also for the way she directly addressed public anger over Gray’s death.

"To the people of Baltimore and the demonstrators across America, I heard your call for ‘no justice, no peace,’" Mosby said. "Your peace is sincerely needed as I work to deliver justice on behalf of this young man. To those that are angry or hurt or have their own experience of injustice, I urge you to channel your energy peacefully."

As she ran for office last fall, Mosby, whose official title is Maryland state’s attorney for Baltimore city, vowed to be tougher on violent crime and more aggressive on police misconduct. Tawanda Jones, whose brother Tyrone West was killed after a violent scuffle with the police, was among those who campaigned forcefully for Mosby — in part, Jones said, to get rid of the incumbent, Gregg L. Bernstein.

"I’m so happy. I’m so excited, I can’t stop crying," Jones said Friday, moments after she saw Mosby’s announcement on television at the preschool where she teaches.

"She gave us her word," Jones said. "I said, ‘How will you handle police brutality?’ She said: ‘If you put me in this chair, I don’t care if they are in uniform or not. I come from a family of officers. Some are good, some are bad. I will hold everybody accountable to the law.’ And thank you, Jesus, she lived it out."

Mosby comes from a long line of law enforcement officers. Her father, mother and grandfather were police officers, and her grandfather, she said, was a founding member of the first association of black police officers in Massachusetts — a point she underscored Friday in comments directed at the Baltimore force.

"To the rank-and-file officers," she said, "please know that the accusations against these six officers are not an indictment of the entire force."

Aside from her family background in law enforcement, Mosby has other personal ties to justice issues. When she was growing up in Boston, a 17-year-old cousin was mistaken for a drug dealer and killed outside her home by another 17-year-old.

Mosby has been outspoken about the need to hold police officers accountable.

While she was campaigning, she said in response to a Baltimore Sun investigation into allegations of police beatings: "Police brutality is completely inexcusable. I’m going to apply justice fairly, even to those who wear a badge."

Mosby is married to Nick Mosby, a Baltimore city councilman, which drew calls from the police union for her to recuse herself. She dismissed those calls Friday at her news conference.

"I uphold the laws. He makes the laws," she said. "And I will prosecute any case within my jurisdiction."

Sonia Kumar, a staff attorney at the American Civil Liberties Union of Maryland, said, "Her actions really told us who she is today."

"For years and years, victims of police violence in our city, overwhelmingly black people, have sought justice for their loved ones to no avail," Kumar said. "This is an historic moment."

When Mosby challenged Bernstein, who is white, in the Democratic primary, police conduct emerged as an issue in some quarters of the city.

"Baltimore voters felt that the incumbent just wasn’t sensitive to their concerns about police issues," said Lester Spence, an associate professor of political science and Africana studies at Johns Hopkins University. "It’s not like they ran on that campaign explicitly, but there was an implicit sense that she was more sensitive to black Baltimore. And this was her moment."

Spence said that he wept when Mosby announced the decision to file charges.

"Black power is about taking the office and using it to make government more humane for black people, and that’s what we see in her," Spence said. "She’s supposed to treat me, the corner boy in the Western District and the police officer exactly the same way when it comes to the law. Historically, because of racism, her predecessors have not effectively done that."

He said Mosby’s move would resonate deeply in neighborhoods that have often felt neglected by political leaders.

"It’s not that she represented the needs of black constituents," he said. "She represented the needs of black, working-class constituents."

Spence added: "Black people have been organizing about this issue for a long time. What this suggests to black people in Baltimore is protests mattered, and their organizing can actually lead to political change. That’s the transformative change."

Outside City Hall on Friday, there was a sense of relief, and of gratitude toward Mosby. U.S. Rep. Elijah E. Cummings, D-Md., who has been in the streets here at night, working to keep the peace between angry demonstrators and the police, said he had complete faith in her.

On Monday, before the riots, Cummings spoke at Gray’s funeral; he gave an impassioned speech asking whether anyone in Baltimore ever really saw, really looked at, young black men like Gray.

"I said: ‘Did you know him? Did you see him? Did anybody see this man who was a mother’s child? Did they see this man who was just trying to get through life? Did they see him as a human being?’" Cummings said Friday, reprising those earlier remarks. "And I have come here today to thank God that Marilyn Mosby and her team saw him."

But even as parts of Baltimore celebrated Mosby’s announcement, criticism emerged about whether she had moved too quickly and whether she had the experience to pursue a high-profile prosecution.

"Think about how long it took Ferguson to go through each and every piece," said Ivan Bates, a former homicide prosecutor in Baltimore who is now in private practice. "It’s easy to charge. It’s hard to convict."

And by filing charges rapidly, Bates said, Mosby set a clear standard for the outcome.

"Just because a medical examiner rules something a homicide, that doesn’t mean you can prove someone’s responsible or guilty for the homicide," Bates said. "If she would have just told the community, ‘Hey, I want to take this before the grand jury,’ I think they would have given her time to do what she needed to do. Now, she’s promised the residents of Baltimore city she’s going to get convictions."

Bates cautioned that Mosby could run into a particular challenge: Any trials could well be moved from Baltimore, perhaps to a community and a jury pool that would be more sympathetic toward the police.

But he said Mosby’s speedy pursuit of charges "fits into her pattern."

"Because of some of her lack of experience, she’s overly aggressive," he said. "A perfect example: There are so many conflicts in this case; she should have gotten out of the case."

Before Mosby appeared Friday to announce the charges, the president of the city’s police union, Gene S. Ryan, had cited allegations of conflicts of interest in asking that Mosby appoint a special prosecutor.

Bates, who had clashed with Mosby when she was an assistant state’s attorney, also accused her of poor management and excessive attention to politics.

"Her lack of experience — it’s almost that she wants to sit down and show everyone she does know what she’s doing," he said. "When you’re the head prosecutor, you almost have to do your job in silence."

Friday’s news, he said, "was an announcement grounded in politics."

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