New York City will settle its long-running legal battle over the Police Department’s practice of stopping, questioning and often frisking people on the street — a divisive issue at the heart of the mayoral race last year — by agreeing to reforms that a judge ordered in August, Mayor Bill de Blasio announced on Thursday.

In making the announcement, which he said he hoped would end a turbulent chapter in the city’s racial history, Mr. de Blasio offered a sweeping repudiation of the aggressive policing practices that had been a hallmark of his predecessor, Michael R. Bloomberg, but that had stoked anger and resentment in many black and Latino neighborhoods. He essentially reversed the course set by Mr. Bloomberg, whose administration had appealed the judge’s ruling.

“We’re here today to turn the page on one of the most divisive problems in our city,” Mr. de Blasio said at a news conference. “We believe in ending the overuse of stop-and-frisk that has unfairly targeted young African-American and Latino men.”

The judge, Shira A. Scheindlin of Federal District Court in Manhattan, found that the department’s stop-and-frisk tactics were unconstitutional, and that it had resorted to “a policy of indirect racial profiling.” At the height of the program, in the first quarter of 2012, the police stopped people — mostly black and Latino men — on more than 200,000 occasions. A vast majority of those stopped were found to have done nothing wrong.

Judge Scheindlin had ordered the appointment of a monitor to develop, in consultation with the parties, widespread reforms of the department’s “policies, training, supervision, monitoring and discipline regarding stop-and-frisk.” That process will go forward as part of the agreement.

The remarkable shift that has occurred in the city’s policing tactics was sharply underscored by those present when Mr. de Blasio made the announcement. Among those standing beside him were some of the Police Department’s harshest critics, namely the directors of the civil rights legal groups that had pursued the two lawsuits that were covered by the agreement.

Mr. de Blasio, in seeking to fulfill a campaign pledge that had helped propel him to his landslide victory, said on Thursday that if the court approved the agreement the city would withdraw its appeal.

The mayor appeared with Police Commissioner William J. Bratton and the city’s corporation counsel, Zachary W. Carter, and chose a symbolic location to make his announcement: the Brownsville Recreation Center in a neighborhood of Brooklyn where the stop-and-frisk tactics had been widely applied.

A 2010 report in The New York Times found that the highest concentration of police stops in the city had occurred in a roughly eight-block area of Brownsville that is predominantly black.

“We will not break the law to enforce the law,” Mr. Bratton said in a statement. “That’s my solemn promise to every New Yorker, regardless of where they were born, where they live, or what they look like. Those values aren’t at odds with keeping New Yorkers safe — they are essential to long-term public safety.”

In discussing the agreement, Mr. de Blasio was generous in his welcome to the city’s former adversaries, who seemed delighted if not disbelieving at the turn of events in a legal battle that began in the late 1990s.

Vincent Warren, executive director of the Center for Constitutional Rights, which helped to handle one of the lawsuits, Floyd v. City of New York, said: “This is where the real work begins. Nobody standing here is pretending this is ‘Mission Accomplished.’ ”

Donna Lieberman, executive director of the New York Civil Liberties Union, which handled the other suit, said the agreement “brings us closer to closing the book on that tale of two cities.”

Bloomberg officials had credited the stop-and-frisk practice for the sharp reduction in murders and the removal of illegal guns from the streets. But while Mr. Bloomberg had characterized Judge Scheindlin’s ruling as dangerous and said it undermined public safety, Mr. de Blasio described the city’s decision to move toward reform as a moment “of profound progress.”

The mayor’s announcement comes amid a steep decline in the number of police stops, to about 21,000, in the third quarter of 2013, when Judge Scheindlin issued her opinion.

Mr. de Blasio said the stop-and-frisk practice was “broken and misused” and cited a “collective commitment to fix the fundamental problems that enabled stop-and-frisk to grow out of control and violate the rights of innocent New Yorkers.”

Indeed, the mayor not only agreed to accept the judge’s findings, but also embraced them. “This is what the democratic process is supposed to do,” he said, “and that includes the judicial process. It’s supposed to bring up the truth of what’s happening in our society, and oftentimes truths that are being ignored.”

In her ruling appointing the monitor, Peter L. Zimroth, a former corporation counsel, Judge Scheindlin did not address how long he would serve. Mr. de Blasio said that as part of the new agreement, the monitor’s role would be limited to three years, “contingent upon us meeting our obligations.”

Mr. de Blasio said he wanted to emphasize that a three-year oversight period was “a shorter window of monitoring than is customary, and that is in part because of our administration’s explicit commitment to reform, including the installation of an independent N.Y.P.D. inspector general.”

Jonathan C. Moore, co-counsel in the Floyd case, said later that the plaintiffs believed the reforms could be achieved within the three-year period.

“And if they drag their feet or they don’t comply, we have the right to ask for more time,” Mr. Moore said. She also ordered a pilot program to outfit a limited number of officers with tiny video cameras that would record while the officers were out on patrol.Among the remedies the judge had ordered were for “erroneous or misleading” police training materials to be corrected, and for the department to revise policies and training regarding racial profiling.

Judge Scheindlin also called for additional reforms, to be developed after members of the community, including the police, were given the chance to be heard at town-hall-style meetings and other forums.

As the deal was described on Thursday, the city formally asked the United States Court of Appeals for the Second Circuit to return the matter to the District Court.

The appeals panel that had blocked Judge Scheindlin’s ruling from going into effect had also removed her from overseeing the case, saying some of her actions could have led “a reasonable observer” to conclude that the appearance of impartiality had been compromised. (The panel found no “misconduct, actual bias or actual partiality” on her part.)

A new judge, Analisa Torres, will be asked to approve the agreement; once it is ratified, Mr. de Blasio said, “we will drop the appeal, and also with the court’s approval, we will settle the case.” The process of developing reforms would then begin.

It appears, though, that the city’s request to send the case to Judge Torres will not be decided immediately.

The appeals court on Thursday gave the police unions that had sought to intervene in the case until Feb. 7 to respond to the city’s request. Patrick J. Lynch, president of one of those unions, the Patrolmen’s Benevolent Association, said his group continued to have serious concerns about how the court-ordered remedies “will impact our members and the ability to do their jobs.”

“Right now we’re in this kind of no-man’s land,” he said. “I need to, as police commissioner, be in a position to say to my officers: ‘This is how you police constitutionally. This is how you police respectfully. This is how you police compassionately. And that these are the guardrails that you have to stay within.’ ”Mr. Bratton made it clear that he hoped the process would now move quickly.

“Police need that guidance,” he added, saying the settlement would provide that. “The quicker we move down this road, the better for all concerned.”