“It was like a tsunami of kids,” said Seth Kaufman, 20, a pizza deliveryman at Olympia II Pizza & Restaurant on South Street. He lifted his shirt to show gashes along his back and arm. He also had bruises on his forehead he said were from kicks and punches he suffered while trying to keep a rowdy crowd from entering the shop, where a fight was already under way.

“By the time you could hear them yelling, they were flooding the streets and the stores and the sidewalks,” Mr. Kaufman said.

The ad hoc gangs have scared many pedestrians off the streets.

City residents are also starting to complain about the number of unsupervised children, and child advocates are asking if there are enough activities to keep young people busy after school.

“We definitely need more jobs for kids, we need more summer jobs for kids, we need more after-school programming, and we need more parent support,” said Shelly Yanoff, executive director of Public Citizens for Children and Youth, a children’s advocacy group in Philadelphia.

Ms. Yanoff added that libraries and after-school programs had been reduced and a program for youth offenders had been cut sharply. On Friday, officials said, two preteenagers assaulted a woman as part of a violent game called “Catch and Wreck,” in which children pick out people who appear homeless and then beat them and take any money they have.

The police, who say these assaults are unrelated to flash mobs, arrested an 11-year-old boy and a 12-year-old girl in the attack. The police said they also planned to charge the boy in an attack on a 73-year-old man who was beaten and robbed in the same area on March 13.

The flash mobs have raised questions about race and class.

Most of the teenagers who have taken part in them are black and from poor neighborhoods. Most of the areas hit have been predominantly white business districts.