The Justice Department on Thursday accused the New Orleans Police Department of systematic misconduct that violated the Constitution, saying that officers used excessive force, illegally arrested people and targeted black and gay residents.

After a 10-month investigation that officials called unprecedented in scope, the Justice Department issued a report saying police engaged in racial profiling against the city’s black majority. From January 2009 to May 2010, for example, officers intentionally fired their weapons at 27 people, the report said. All 27 were African American.

In releasing their findings at a news conference in New Orleans, Justice officials vowed to work with the city to reform the scandal-plagued department, whose reputation has further deteriorated since Hurricane Katrina. The government is not suing New Orleans but plans to implement reforms through a court-ordered settlement.

“The problems facing the New Orleans Police Department are serious, wide-ranging, systemic and deeply rooted in the culture of the Department,” said Thomas E. Perez, assistant attorney general for the civil rights division, which led the probe.

“My colleagues and I are not naive about the enormity of the task that lies ahead,” Perez said. “But we at the Justice Department have been heartened by the dedication to sustainable change we have seen throughout the city.”

In an unusual move, New Orleans officials had invited in the federal government to help remake the police department, after years of complaints by residents that escalated after Katrina devastated the city in 2005.

City leaders, who appeared with senior Justice Department officials at the news conference, said they welcomed the report and vowed to use it to change the police department’s culture.

“The findings are sobering and the challenges ahead are daunting, but we will do whatever it takes to make this right,’’ said Mayor Mitch Landrieu, who triggered the investigation when he wrote a letter last year to Attorney General Eric H. Holder Jr.

Landrieu said police officials cooperated with federal investigators, and police Superintendent Ronal Serpas said officers “are committed to change. . . . This is a marathon, not a sprint, and every member of the department, from me on down, are committed to improving.”

Justice Department investigators did not examine the widely reported allegations of police misconduct after Katrina, confining their review to the past two years. They found that the police department has “long been a troubled agency” and that it suffers from inadequate training, supervision and community oversight.

The report said that police “routinely use unnecessary and unreasonable force,’’ including against mentally ill people and suspects already in handcuffs, and that offending officers are rarely punished. The department’s police dogs were “uncontrollable to the point where they repeatedly attacked their own handlers,’’ the report said.

Numerous community members, especially African Americans and gay men and lesbians, told investigators about alleged police harassment and unjustified stops and arrests. Many officers echoed those concerns, the report said.

The racial disparity, investigators said, was particularly striking, even in a city that is 60 percent African American. In 2009, police data showed that officers arrested 500 African American males younger than 17 for offenses ranging from homicide to larceny. During the same period, eight white males of the same age group were arrested.

Although there is also a racial disparity in arrest data nationwide, it is “not nearly as extreme” as in New Orleans, the report said

The Justice Department probe began May 15, less than two weeks after Landrieu’s letter to Holder. In a separate criminal investigation, 20 officers have been charged since last year in connection with the killing of civilians, and eight have been convicted or pleaded guilty.

In June, five officers were charged in connection with the death of Henry Glover, whose burned body was found in a car near a police station shortly after the hurricane. Three have been convicted.

Eleven officers were also charged in connection with the killing of two unarmed civilians on the Danziger Bridge. Five have pleaded guilty.

Justice Department officials said the civil investigation was unusual in the level of engagement between investigators and the New Orleans community. The department participated in more than 40 community meetings, and officials said they plan to meet with additional residents in the coming months.

Federal officials plan to negotiate a consent decree with the city that will set benchmarks on use of force, racial profiling and other violations “so we can measure progress on a regular basis,’’ Perez said.