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In the District of Columbia, about 20 teenagers entered the G-Star Raw store on Connecticut Avenue at Dupont Circle in April and stole about $20,000 worth of merchandise before fleeing.

The problem has given a heavy dose of bad publicity to Philadelphia, which depends on tourism dollars. At one point last summer, city police stationed four officers at each intersection on weekend nights in a neighborhood where youths had congregated.

Mr. Nutter, a strong ally of President Obama and vice president of the U.S. Conference of Mayors, took to the pulpit of his Baptist church Sunday to deliver a 30-minute sermon about black families taking responsibility for the behavior.

“The Immaculate Conception of our Lord Jesus Christ took place a long time ago, and it didn’t happen here in Philadelphia,” Mr. Nutter said. “So every one of these kids has two parents who were around and participating at the time. They need to be around now.”

The mayor told parents, “If you’re just hanging out out there, maybe you’re sending them a check or bringing some cash by. That’s not being a father. You’re just a human ATM. … And if you’re not providing the guidance and you’re not sending any money, you’re just a sperm donor.”

At City Hall on Monday, the mayor said gangs will be confronted.

“If you want to be aggressive, we’re going to be aggressive,” Mr. Nutter said, before sending a warning.

“Let me just share this with you. We’ve got the biggest, baddest gang in town - a committed group of citizens and a committed government - and we’re working together and we’re not going to have this nonsense anymore.”