(CNN) -- Philadelphia has been plagued by teen violence, but the City of Brotherly Love is fighting back.

Mayor Michael Nutter announced this week a robust initiative that began with a stiff curfew at 9 p.m. Friday.

The effort comes after a string of attacks on residents by groups of young people who are alerted to sudden gatherings at a given place via e-mail and social media.

"It's a growing problem in this country, police Commissioner Charles H. Ramsey told CNN Friday.

Nutter delivered tough remarks about the problem in a church sermon Sunday that has received national and international attention, a blunt no-excuses scolding that happened to coincide with the start of the England riots.

An African-American, Nutter noted that those involved in the Philadelphia attacks are predominantly black and said their behavior damaged themselves and damaged their race.

He upheld religious values, underscored the importance of parental discipline and lambasted absentee parents, particularly fathers, for neglecting and not keeping watch over their children.

He said fathering is engaging with the child and shaping them. He described a neglectful dad as a "human ATM" or a "sperm donor."

Accordingly, his measures target both the young attackers and their parents or legal guardians.

Nutter signed an order temporarily moving the curfew to 9 p.m. Fridays and Saturdays for everyone under the age of 18 in targeted enforcement districts.

They are in Center City, the downtown area, and University City, the West Philadelphia neighborhood where the University of Pennsylvania is located. Throughout the rest of the city, the curfew will remain 10 p.m. for those under the age of 13 and those for minors under the age of 18.

"Minors who are caught breaking curfew will be sent home, brought home or transported to a police station where their parents will be contacted. Minors may be issued a citation with a $100 to $300 fine for a first offense," the city said in a press release.

Parents will receive a first-violation notice and then will be fined up to $500 for successive violations.

"These notices and citations will be issued when the parent comes to collect their child from the police station."

If parents don't come for their child "within a reasonable time, police will contact the Department of Human Services to start an investigation."

Parents whose child under the age of 18 "is found liable or guilty by a court for injury, theft or other criminal acts" will be "liable to the person who suffered the injury or loss of property."

Parents and legal guardians will be "subject imprisonment for up to 90 days for repeat curfew violation offenses."

Ramsey said Friday the first offense will most likely prompt a warning, with fines to follow for other curfew violations. "If they come down and start to assault people or damage property we are going to arrest them."

Up to the beginning of the school year next month, police will maintain a greater presence in the targeted enforcement areas and work to improve camera surveillance. It will also expand weekend hours at recreation centers.