NEW YORK (AP) - A 24-year-old non-profit worker wept on the witness stand on Tuesday as he described being handcuffed by police near his home - one of about 5 million stops by New York City police over the past decade of mostly black and Hispanic young men.

Mr Nicholas Peart, who is black, is one of about a dozen New Yorkers expected to tell their stories of being stopped, questioned and frisked by police in a federal trial challenging how police use the tactic.

US District Court Judge Shira Scheindlin, who has said in earlier rulings that she is deeply concerned about the practice, has the power to order reforms that could bring major changes to the largest US police force.

Lawyer Darius Charney of the Centre for Constitutional Rights which filed the suit in 2008 on behalf of four New Yorkers, called many of the stops a "frightening and degrading experience" that violates the civil rights of many residents.

The class-action lawsuit seeks broad reforms.

The mayor and police commissioner say the practice is a life-saving, crime-stopping tool that has helped drive crime down to record lows. Police go where the crime is - and crime is overwhelmingly in minority neighbourhoods, they said, and officers do not targeting people solely because of their race.

"The New York Police Department is fully committed to policing within the boundaries of the law," said Ms Heidi Grossman, an attorney for the city. "Crime is not distributed evenly across the city."

About half the people who are stopped are subject only to questioning. Others have their bag or backpack searched. And sometimes police conduct a full pat-down. Only about 10 per cent of all stops result in arrest, and a weapon is recovered a small fraction of the time.

The tactic is legal. Officers are allowed to make stops based on "reasonable suspicion," a less rigorous standard than "probable cause" needed to justify an arrest. Police are required to fill out a form where they check off the justification for stopping someone, which includes, "furtive movements," suspicious bulges and if someone fits the description of a crime suspect.

City lawyers have sought to discredit witnesses so far by suggesting that their stories had evolved over the years to become more dramatic, and their memories were faulty.

The trial is expected to last more than a month.

Mr Peart, who is the guardian for his three siblings after his mother died of cancer, testified that he was stopped three times by police, starting on his 18th birthday. But a stop in 2011 reduced him to tears.

He testified that he was walking to the corner store one night to get milk when officers stopped him, handcuffed him and put him in the back of a car. One officer took his keys and went into his building. Mr Peart said he was concerned because he didn't know how his siblings would react if the officer knocked on the door.

"I was afraid he would go into my apartment, and I wasn't there to take care of the situation," he said.

Eventually the officer returned, and he was freed.

"To be treated like that, by someone who works for New York City, I felt degraded and helpless," Mr Peart said.