There have been several assaults by teens on residents in recent weeks, and the beatings have left people badly injured. The city cites the culprits as members of a "flash mob," which is a group of people who decide to gather at a given place via e-mail and social media.
Nutter, who is African-American, delivered a frank sermon at his church, Mount Carmel Baptist in West Philadelphia, on Sunday about the problem.
The mayor lashed out at the attackers, saying "you've damaged yourself, you've damaged another person, you've damaged your peers and, quite honestly, you've damage your own race."
On Monday, Nutter signed an order reducing curfews to 9 p.m. on Fridays and Saturdays for people under the age of 18 in Center City, the heart of Philadelphia's downtown, and University City, a neighborhood to the west where the University of Pennsylvania and Drexel University are located.
More here: Philly mayor: 'No excuses' for flash mob attacks - CNN.com