The New York Police Department's practice of stopping, questioning and frisking people on the street is facing its biggest legal challenge with a federal civil rights trial on whether the tactic unfairly targets minorities.

Police have made about 5 million stops of New Yorkers in the past decade, mostly black and Hispanic men. The trial ill include testimony from a dozen people who say they were targeted because of their race and from police whistleblowers who say they were forced into making slipshod stops by bosses who were too focused on numbers.

U.S. District Court Judge Shira Scheindlin, who has said in earlier rulings that she is deeply concerned about stop and frisk, is not being asked to ban the tactic, since it has been found to be legal, said ABC News. But she does have the power to order reforms, which could bring major changes to how the nation's largest police force and other departments use the tactic.

New York Mayor Michael Bloomberg and Police Commissioner Raymond Kelly say it is a necessary, life-saving, crime-fighting tool that helps keep illegal guns off the street and has helped New York reach all-time crime lows, said ABC News.

Recent polls show a stark divide over how blacks and whites view the tactic, while a majority or plurality of Hispanics approve of it, ABC News. The debate has drawn in Muslim-Americans concerned about NYPD surveillance revealed in a series of reports by The Associated Press.

The trial is expected to last more than a month and include more than 100 witnesses.