A US Federal Judge found that stop-and-search tactics used by the New York Police Department have violated the constitutional rights of tens of thousands of citizens and are racist, and called for a federal monitor to oversee reforms to the policy.

Judge Shira A. Scheindlin ruled that police officers have been systematically stopping innocent people in the street without any objective evidence that they had been committing an offence. Cops usually searched young black and Latino men for weapons or drugs before letting them go.

The ruling follows a more-than-two-month non-jury trial. The 195 page decision found that in 88 percent of ‘stop and frisks’ the police ended up letting the person go without an arrest or a ticket.

The judge said this percentage was so high it suggested there was no credible reason to suspect someone of criminality in the first place.

She found that the stop-and-frisk-episodes had demonstrated a widespread disregard for the Fourth Amendment, which protects against unreasonable searches and seizures by the government, and also violated the 14th Amendment, which addresses citizens’ equal rights and protection under the law, regardless of their race or ethnicity.

Scheindlin also ruled that she would designate an outside lawyer, Peter L. Zimroth, a former corporate counsel and prosecutor in the Manhattan district attorney’s office, to monitor the NYPD’s compliance with the Constitution. This will leave the New York police under a degree of judicial control that will doubtless shape policing policies under the next mayor.

“Far too many people in New York city have been deprived of this basic freedom far too often. The NYPD’s practice of making stops that lack individualized reasonable suspicion has been so pervasive and persistent as to become not only part of the NYPD standard operating procedure, but a fact of daily life in some New York City neighborhoods,” she said.

She added that the plaintiffs who had instigated the case “readily established that the NYPD implements its policies regarding stop and frisk in a manner that intentionally discriminates based on race.”

The stop-and-frisk incidents are part of incumbent Mayor Michael Bloomberg’s tough crime-fighting legacy, and although the number of people searched has soared over the last decade, crime has continued to fall since the 1990s.

Scheindlin heard evidence from a dozen black, Latino or biracial people who had been stopped by police as well as from statistical experts who had examined police paper work detailing some 4.43 million stops between 2004 and the middle of 2012. A number of police officers and commanders also gave evidence; typically they defended their own actions saying they only made the stops when they thought criminal activity was occurring.

The judge found that the New York police had overstepped their authority to briefly stop and investigate people who are behaving suspiciously and that in effect they were watering down the legal minimum standard required to stop someone.

Legal experts said that this was the largest and broadest-sweeping case against the US’s largest police force, and that this ruling may have an effect on how other police departments conduct street stops.

New York Mayor Michael Bloomberg has announced that he will appeal Judge Scheindlin's ruling.