BALTIMORE — Mayor Stephanie Rawlings-Blake, who has come under withering criticism since riots tore through this city in April, announced Friday that she would not seek re-election next year, a surprise development that came just as Baltimore was preparing for the racially charged trials of six police officers charged in the death of a 25-year-old black man, Freddie Gray.

A somber Ms. Rawlings-Blake, 45, who would have faced a crowded field of potential challengers, made her announcement at a hastily convened news conference in her ceremonial conference room at City Hall here.

The mayor said she had been thinking about the decision — and praying about it — for two months and had become convinced that she needed to focus on healing the city and reforming its Police Department.

She realized that any time she spent on re-election “was time that I would be taking away from my current responsibility to my city, to the city that I love, the city that I took an oath to serve,” she said. “And because of that, I have made the decision not to run for re-election.”

She spoke of the city’s wounds that need tending, particularly the relationship between the police and the community, and said that as a candidate for re-election, she would run the risk of her every move being seen through a political lens.

When calling donors or conferring with political strategists, “I constantly had the feeling that I should be back at City Hall. I knew that I needed to spend time, the remaining 15 months in my term, focusing on the city’s future, and not my own,” she said.

Her decision came one day after a judge ruled that the trial of the officers would remain in Baltimore and in the same week that the city announced a settlement of $6.4 million with the family of Mr. Gray, who died after suffering a spinal cord injury in police custody. The death spawned a wave of looting, arson and riots — the worst unrest the city had seen since 1968.

The mayor, a Democrat, is leaving elective office just as her national star is rising; this summer, she became the first black woman to head the United States Conference of Mayors. But her tenure in Baltimore has been rocky, especially since the unrest that followed Mr. Gray’s death, and several candidates are vying to succeed her.

One is Sheila Dixon, a popular former mayor who was forced out of office in 2010 because of a scandal and who has already announced she is running. Another potential candidate is Nick Mosby, a Baltimore city councilman and the husband of Marilyn J. Mosby, the state’s attorney for the city of Baltimore, who is prosecuting the officers.

Both Ms. Dixon and Mr. Mosby issued statements thanking the mayor for her service — even before her announcement was made. Ms. Dixon said the mayor had “earned the right to pursue other goals,” and she promised to “pray for the Rawlings-Blake family.”

Mr. Mosby — who has not announced his candidacy but said in an interview that he was “seriously considering” running — was openly critical of the mayor’s tenure. “Baltimore needs a new direction,” he said.

Last week, two other mayoral hopefuls — state Senator Catherine E. Pugh, who challenged Ms. Rawlings-Blake unsuccessfully in 2011, and City Councilman Carl Stokes — announced that they would run.

Ms. Rawlings-Blake insisted that she had no doubt that she would win re-election if she ran, and she said she had more campaign money than any of her opponents.

“I haven’t lost an election since middle school,” she said. “It’s not that I didn’t think I could win, it’s that I had to ask myself the question: ‘At what cost?’ ”

Circumstances, she said, dictated her decision.

“You don’t get to choose; you play the cards you’re dealt,” she said. “The city is in a very critical time right now. We’re working very hard to reform the Police Department. We have to get our city through six separate trials. Nobody planned for that.”

Viewed as aloof by her constituents, Ms. Rawlings-Blake has been widely criticized for allowing protests over Mr. Gray’s death to get out of hand before asking Maryland’s governor, Larry Hogan, to call in the National Guard — and especially for a comment that she made at a news conference in which she said she had instructed the police to make sure that “protesters were able to exercise their right to free speech.”

Ms. Rawlings-Blake grew up around politics and civil rights activism. Her father, Howard Rawlings, who was known as Pete, was a civil rights advocate who became the first black chairman of the powerful appropriations committee in Maryland’s House of Delegates. When she was a little girl, friends say, Ms. Rawlings-Blake would race through the corridors of the State House in Annapolis, telling her parents she wished they could live in the capital city full time.

With her father’s help, Ms. Rawlings-Blake, a graduate of Oberlin College and the University of Maryland law school, became, at 25, the youngest City Council member in Baltimore history. While on the Council, she also worked as a Legal Aid lawyer and a public defender.

Eventually, she became the Council president. In 2010, when Ms. Dixon was forced to resign, Ms. Rawlings-Blake stepped in as mayor. She won election in her own right the next year.

On Friday, Ms. Rawlings-Blake declined to answer questions about whether she might return to politics in the future and make a run for higher office.