The attacks, the second of their kind in just over a month, have sparked fear, frustration, and concern that the next outburst could kill someone.
The outrage prompted officials to flood downtown with police officers this weekend and drew a pledge for a long-term comprehensive response from Mayor Nutter, expected to be announced Monday.
Police also will strictly enforce a curfew this weekend: Children younger than 13 must be home by 10 p.m.; those younger than 18 by midnight.
The attacks have left people struggling to understand the brazenness and explosiveness of the teens' violent behavior.
"Typically, when doing juvenile crime you can see a reason for an attack. There's an aggression, there's a slight, something that sets someone off," prosecutor Leslie Gomez said after the Friday hearing. "These were completely vicious and random attacks on strangers, one after the other after the other."
Aleek Hamilton, 19, was one of three teens arrested in the June 25 attacks on 15th Street, which occurred after hundreds of youths left the Susquehanna Community Festival and headed to LOVE Park and South Street.
"Some people go down there for the girls; others just to be seen," Hamilton said in an interview, adding that he was in the crowd but did not participate in the violence. "Some people go and try to be all tough until the police come, then they run."
Some kids just don't care, he said.
"They get greedy. They see someone's iPod or slick phone and they want it and they just take it," he said.
Those kids want to make a name for themselves, he said.
Jeremy Schnekel, 23, was trying to catch a train home to Northeast Philadelphia when he became the first victim on July 29. Around 9 p.m. near 15th and Sansom Streets, he saw a pack of young people running toward him.
He said that he tried to let them pass but that one of them punched him in the head and others pounced on him, beating him to the ground. Police said the 11-year-old tried to rip Schnekel's leather briefcase from his neck.
They were laughing as they beat him, Schnekel said. "It looked like they all thought it was fun."
The recent violence does not fit the pattern of the flash mobs of 2009 and 2010 that were organized through social media, police say.
This summer's attacks have been committed by primarily black teens, a small number of whom turn Center City excursions into random acts of violence. In recent months, Chicago has dealt with similar teen attacks.
Police and prosecutors here said that there was no evidence the attacks were racially motivated but that they were crimes of opportunity.
Explanations for the attacks vary. Some blame parents. Some blame schools. Some business leaders say policing levels have not risen proportionately with the growth of Center City. In 2002, there were 6,851 police officers citywide; today, there are 6,646.
A teen cousin of one of the youths arrested in the July 29 attacks offered his opinion but not his name after the hearing.
"They acting out their anger," he said of the teens. "It ain't going to stop."
Philadelphia Police Commissioner Charles H. Ramsey said in a recent interview that "people want easy solutions to complicated problems. Parenting is a part of this but not all of it. . . . It's not all the schools' fault. These kids know what they're doing. . . . They should face strict punishment."
In the short term, Ramsey said, residents and visitors will notice a substantial downtown police presence, including extra weekend patrols on South Street and in Center City and Old City.
"Anyone who comes to this city right now is going to see an awful lot of police," he said.
More officers will be permanently assigned to Center City, he said. Those assignments are in addition to officers reassigned to the area after last year's flash mobs.
Ramsey acknowledged that the public may question whether additional patrols will work.
"The mayor's plan will have a lot of components that go beyond police deployment," he said. "But right now, we're responsible for apprehending people who commit criminal acts."
On Thursday, police and business representatives met at the Union League to discuss collaborative efforts to prevent such attacks.
Paul Levy, head of the Center City District, said the meeting was a necessity.
He said the district had moved more of its community representatives - the people in the turquoise jackets - to the night shift to assist police by communicating any signs of trouble.
A highly visible downtown police presence is the best temporary solution, he said.
"Right now, there is a gap in the fabric of safety and the message has to be sent that this behavior won't be tolerated," he said.
A Common Pleas Court judge will hear Hamilton's case in September.
Police said Hamilton was walking in a large crowd of teens along 15th Street on June 25 when he reached through the patio window of a restaurant and grabbed a woman's cellphone.
Another woman gave chase, police said, and a female teen punched her in the face. Hamilton spent three weeks in jail after an eyewitness identified him as the phone thief.
In an interview Thursday, he said he was walking in the crowd with his brother and some girls they had met on their way from the concert. He said that he saw others acting out but that he did not.
His experience offers a glimpse into the mob mentality and a warning to those who may want to maraud through Center City.
Hamilton was polite and friendly as he told his story on the steps of his mother's North Philadelphia rowhouse. He wore his Mastbaum High School football shirt. He graduated the week before his arrest and said his troubles had stalled his plan to attend Lincoln Technical Institute, where he wants to study business.
He described the night:
"There was a pack of boys in front us going crazy, making lots of noise, doing stupid stuff," he said. Some girls were, too, he said. "They do it to fit in."
He saw the commotion but continued to South Street, where officers on bike patrol stopped him, he said.
"They told me I fit a description," he said. After one of the victims identified him, he was put in a police wagon.
"I'm sick of these flash-mob kids," he remembered a police officer saying.
He spent his first night in jail in a crowded holding cell with a broken toilet.
"This is not a place I want to be," he remembered thinking.
He was transferred to the House of Corrections.
For breakfast, "nasty salami, watery oatmeal, and half-cooked potatoes." For lunch, "more salami."
"Sleeping was horrible," he said, with all the other prisoner's screams.
He wore his boxers in the shower.
Hamilton said some teens go to Center City to rob and beat people for a simple reason.
"They do it to be known," he said. "I think it's immature to do something so stupid just to be known."
Contact staff writer Mike Newall at 215-854-2759, firstname.lastname@example.org, or MikeNewall on Twitter.