Police spokesman Lt. Raymond Evers said it's anyone's guess whether the city can expect more roaming bands of young people.
"They don't give us their schedule for the summer; if they did we'd be right on top of them," Evers said. "A lot of these things are impromptu. That's why it's so hard to police them."
Deputy Commissioner Kevin Bethel said that police are closely monitoring social media for any word of such gatherings and that officers are working with some of the youths who were arrested for similar activities in 2009 to better understand such events.
For his part, Police Commissioner Charles Ramsey called on parents to be aware of what their children are doing.
"If they get caught and they get charged, they're going to have a problem," he said.
"We're going to push very hard with the District Attorney's Office and the courts to see to it that there's a severe penalty."
Just last week, a 23-year-old man was sentenced to five to 20 years in prison for participating in a May 2009 "flash mob" in Center City in which a bicyclist was severely injured.
Police made three arrests in the robbery cases from Saturday night, but they have no leads on the assaults, including the attack on Emily Guendelsberger, 27, city editor for local arts and entertainment content for the Onion, who remained hospitalized yesterday with a broken leg.
Meanwhile, state Rep. Jewell Williams, one of the founders of the Susquehanna Festival, which promotes peace and community, said he found it hard to believe that the group that ended up on South Street began at the festival three miles away.
Either way, Williams said he was upset to hear of the group's activities. He said the best way to curb such actions by groups of young people is to stop giving them names like "flash mobs."
Although the term typically refers to groups organized through social media for zany events like public pillow fights, starting in the spring of 2009 in Philadelphia, the term came to describe groups of youths who organize over social media and text messaging to riot.
"When you give a name and a title to any movement, whether it's peaceful or not, you've now organized people," Williams said. "It's all in the name. Take away the name and you start breaking people down and dispersing the group."
Police officials also stayed away from the term "flash mob" yesterday. Ramsey called the group a "roaming band of young people."
Other areas have also been experiencing issues with unruly teens this year.
In Chicago, violent groups of youths have been attacking people with baseball bats and robbing them of electronics. Just last Tuesday, about 50 teens broke into a Chicago Walgreens and stole food and drinks.
On Thursday in Upper Darby, police said about 40 teens from West Philly stole thousands' worth of merchandise from a Sears store. Of the 16 who were arrested, the youngest was 11 and the eldest, 19.