PHILADELPHIA — Six people, including an off-duty police detective and his wife, were injured after a “flash mob” attack by some among a crowd of juveniles in downtown Philadelphia, police said.

Police said a large crowd of juveniles were at 16th and Walnut streets, a popular spot for dining and shopping, at about 6 p.m. Saturday when some people began randomly assaulting people on the street. News of the assault comes after several were arrested in October for similar assaults by mobs of juveniles near Temple University. A witness to that attack said he overheard police saying the kids were playing “the knockout game.”

In Saturday’s attack, a 55-year-old off-duty police detective saw a 20-year-old man and a 21-year-old woman being assaulted and tried to arrest one of the offenders. He was punched from behind by several people, police said.

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The detective’s 53-year-old wife splashed water on the offenders to try to stop them from attacking her husband, and she was punched in the face, police said. All of the offenders then fled. The four victims were taken to a nearby hospital.

Immediately afterward, there was another assault by several juveniles nearby, and two 16-year-old youths were arrested, police said.

In all, six people were injured.

The police detective had an orbital fracture to his right eye while the others had minor injuries.

One man told WPVI-TV that he feared for his safety when the large group of teens appeared, and he saw other people running into nearby stores to escape.

“It’s crazy out here,” said Dwight Magood. “I don’t know if they were trying to protest or ‘flash mobbing.’ But it’s not the right way to do it, whatever it was.”