NEW YORK — The New York Police Department deliberately violated the civil rights of tens of thousands of New Yorkers with its contentious stop-and-frisk policy, and an independent monitor is needed to oversee major changes, a federal judge ruled Monday in a stinging rebuke of what the mayor and police commissioner have defended as a life-saving, crime-fighting tool.

U.S. District Judge Shira Scheindlin said she was not putting an end to the policy, but rather was reforming it. She named an independent monitor who would develop reforms to policies, training, supervision, monitoring and discipline. She also ordered that officers test out body-worn cameras in the police precinct where most stops occurred.

“The city’s highest officials have turned a blind eye to the evidence that officers are conducting stops in a racially discriminatory manner,” she wrote. “In their zeal to defend a policy that they believe to be effective, they have willfully ignored overwhelming proof that the policy of targeting ‘the right people’ is racially discriminatory.”

Mayor Michael Bloomberg vowed to appeal the ruling.

“We didn’t believe that we were getting a fair trial,” the mayor said. “And this decision confirms that suspicion.”

NYPD Commissioner Raymond Kelly said the notion of racial profiling is “recklessly untrue.”

For years, police brass had been warned that officers were violating rights, but they nevertheless maintained and escalated “policies and practices that predictably resulted in even more widespread Fourth Amendment violations,” Scheindlin wrote.

“Far too many people in New York City have been deprived of this basic freedom far too often,” she said.

“The NYPD’s practice of making stops that lack individualized reasonable suspicion has been so pervasive and persistent as to become not only a part of the NYPD’s standard operating procedure, but a fact of daily life in some New York City neighborhoods.”

Four men sued the department in 2004, saying they were unfairly targeted because of they were minorities. Scheindlin issued her ruling after a 10-week bench trial in which she found that nine of the 19 stops discussed at the trial were unconstitutional, and an additional five stops included wrongful frisking.

Stop-and-frisk is a constitutional police tactic, but Scheindlin concluded that the plaintiffs had “readily established that the NYPD implements its policies regarding stop and frisk in a manner that intentionally discriminates based on race.”

City lawyers had argued the department does a good job policing itself with an internal affairs bureau, a civilian complaint board and quality assurance divisions.

The judge rejected their arguments.