WASHINGTON — Baltimore’s top law enforcement and political leaders vowed a sweeping overhaul of the city’s police force after the Justice Department issued a searing rebuke of the agency’s practices, which federal authorities say regularly discriminated against black residents in poorer communities.

Officials warned, however, that changing a department entrenched in a culture of unconstitutional policing would be a slow process and could cost millions.

‘‘Police reform won’t happen overnight or by chance,’’ Principal Deputy Assistant Attorney General Vanita Gupta said at a news conference unveiling the findings of the report Wednesday. ‘‘It’s going to take time and it’s going to require a focused and sustained effort.’’

Gupta said there were ‘‘longstanding systemic deficiencies’’ within the Baltimore Police Department and that ‘‘sustainable reform’’ was necessary to keep both officers and the community safe.

The sharp indictment of the agency came in an extensive report the federal government released this week after a 14-month ‘‘pattern or practice’’ investigation of the city’s police force.

The inquiry found that a police force rooted in ‘‘zero tolerance’’ enforcement that started in 1999 but ended a decade ago has created a deep divide between police and many members of the community it serves. The city’s policing strategy, lack of training, and inattention to officer accountability has cultivated an agency that allows and encourages officers to stop, arrest, or search black residents with little or no legal justification. The report also found that officers engaged in unnecessary force against juveniles, people with mental health issues, and people who were restrained and presented to no threat.

‘‘BPD deployed a policing strategy that, by its design, led to differential enforcement in African-American communities,’’ the report stated. ‘‘But BPD failed to use adequate policy, training, and accountability mechanisms to prevent discrimination, despite longstanding notice of concerns about how it polices African-American communities in the city.’’

In other words, according to the 163-page Justice Department report: ‘‘The relationship between the Baltimore Police Department and many of the communities it serves is broken.’’

To many in the African-American commuity, the report was familiar reading.

Danny Marrow, a retired food service worker, said that over the years, he has been stopped and hassled repeatedly by police.

‘‘It started when I was 8 years old and they’d say, with no probable cause, ‘Hey, come here. Where are you going?’ ’’ he said. ‘‘No cause, just the color of my skin.’’

Mayor Stephanie Rawlings-Blake said while the findings of the report are ‘‘challenging to hear,’’ the investigation creates a ‘‘crucial foundation’’ that will allow Baltimore to change the department.

‘‘It’s so very important that we get this right,’’ Rawlings-Blake said.

Now that the investigation is complete, city officials will work with the Justice Department to implement a series of court-mandated reforms outlined in what is known as a ‘‘consent decree.’’ The mayor said it could cost the city anywhere from $5 million to $10 million annually to make the suggested changes, which include improved training programs and new technology and equipment to modernize the police force.

The court-enforced order will be independently monitored and designed to sustain reform regardless of who is the police commissioner or mayor, justice officials said.

City police Commissioner Kevin Davis said that he has already fired some officers as a result of the Justice Department’s investigation. Davis also said that he would not tolerate policing that is sexist, racist, or discriminatory.

►Elderly woman fatally shot during police event. A13

Material from the Associated Press was used in this report.