Under Philadelphia's curfew ordinance, those under 13 must be off the streets by 10 p.m. and those from 14 to 17 by midnight. Nutter, in a statement, reminded city residents that teens and their parents can both face penalties for curfew violations.
"I want to strongly encourage parents and guardians to be vigilant and to look out for their children this weekend," he said. "There is no excuse for young people to be able to participate in coordinated, violent behavior if parents are doing their job. It is your responsibility, not the government's, to watch your kids."
The city has increased patrols in Center City where two men were robbed and beaten. One of those arrested was 11.
Police said the young people in the mob appeared to be communicating by text message.
Nutter said he plans a news conference Monday to announce additional steps to curb teen violence.