Six people were hurt, including an off-duty police officer and his wife, in what people called a "flash mob" attack in Center City on Saturday night.
According to police, a small group of juveniles broke off from a large crowd and began attacking people in the area of 16th and Walnut Streets around 6 p.m.
The group first went after a 21-year-old man, police said, which led the off-duty detective to try and intervene. The perpetrators ran away, police said.
Then the detective, whom police did not identify but said was 55 years old, saw two women - ages 20 and 21 - getting kicked by some juveniles. When he attempted to arrest one of them he was "punched from behind by several unknown persons," police said.
The detective's wife, 50, tried to splash water on the attackers to get them to stop, but she was then punched in the face, police said.
Immediately after that assault, a 28-year-old man was assaulted around the corner, on the 100 block of South 16th Street, police said.
The detective, his wife, and the two women were treated at Jefferson Hospital. The detective had an orbital fracture of his right eye; the others suffered minor injuries, police said.
Authorities said two youths, both 16, were arrested. Investigators were looking for surveillance footage to help gather evidence to identify more potential suspects.
Two weeks ago, four teens were arrested in a similar "flash mob" incident near Temple University's main campus in North Philadelphia. That attack, which included as many as 150 juveniles, left at least one student hospitalized amid wide-ranging violence that police said included rocks being thrown at cars and a police horse being punched.