On a two-block stretch of Brownsville in April, the police stepped aside and let residents respond to 911 calls. It was a bold experiment that some believe could redefine law enforcement in New York City.

It had been a quiet April afternoon until about a dozen teenagers began running up Pitkin Avenue in Brownsville, yelling and cursing. They were chasing a girl of about 14 and it was clear they wanted a fight.

Five plainclothes police officers watched warily. Across Pitkin stood about half a dozen men, civilians in jeans and purple-and-gray sweatshirts.

“They got it,” an officer said.