(CNN) -- Philadelphia's mayor has extended the city's early juvenile curfew through Labor Day weekend, his office announced Tuesday.
The curfew will remain in place until school starts, when the stricter curfews already in place kick back in, said Mark McDonald, Mayor Michael Nutter's press secretary.
The tighter summer curfew went into effect August 12.
"We had about 70 curfew violations over the course of (the first) Friday and Saturday nights, and I am sure that young people and their parents have now gotten the message," Nutter said last week.
Nutter announced the earlier curfew following a string of mob attacks by young people alerted to gatherings via e-mail and social media. The new city ordinance mandated a 9 p.m. curfew on Fridays and Saturdays for all minors under 18 in Center City and University City.
In June, a woman and her male companions were attacked in Philadelphia by a group of about 40 teens, leaving her with a broken leg. Another man was attacked the same night a few blocks away.
Four suspects -- ages 11, 16, 17 and 19 -- were arrested for assaults in July, and another three juveniles turned themselves in to authorities in early August after assaulting a man last month.
On the first Saturday night the curfew was in effect, Nutter spent the evening bowling with teens at an event planned to provide some alternatives for entertainment.
The mayor also said he has ordered expanded hours of service at 20 Philadelphia recreation centers.
Outside the two special enforcement areas, the normal summer curfew is in effect and children under 13 must be home by 10 p.m. while young people under 18 must be home by midnight, the mayor said.
"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 news release.
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 will receive a notice for first violations, and will be fined up to $500 for successive violations. They are also subject to imprisonment for up to 90 days for repeat curfew violation offenses, according to the city.
In addition, 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," the statement said.