In a repudiation of a major element in the Bloomberg administration’s crime-fighting legacy, a federal judge has found that the stop-and-frisk tactics of the New York Police Department violated the constitutional rights of tens of thousands of New Yorkers, and called for a federal monitor to oversee broad reforms.

In a decision issued on Monday, the judge, Shira A. Scheindlin, ruled that police officers have for years been systematically stopping innocent people in the street without any objective reason to suspect them of wrongdoing. Officers often frisked these people, usually young minority men, for weapons or searched their pockets for contraband, like drugs, before letting them go, according to the 195-page decision.

These stop-and-frisk episodes, which soared in number over the last decade as crime continued to decline, demonstrated a widespread disregard for the Fourth Amendment, which protects against unreasonable searches and seizures by the government, according to the ruling. It also found violations with the 14th Amendment.

To fix the constitutional violations, Judge Scheindlin of Federal District Court in Manhattan said she intended to designate an outside lawyer, Peter L. Zimroth, to monitor the Police Department’s compliance with the Constitution.

The decision to install Mr. Zimroth, a partner in the New York office of Arnold & Porter, LLP, and a former corporation counsel and prosecutor in the Manhattan district attorney’s office, will leave the department under a degree of judicial control that is certain to shape the policing strategies under the next mayor.

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