Authorities in the US city of Philadelphia have ordered a weekend curfew for minors in an effort to halt a series of violent flash mobs.
Mayor Michael Nutter said that the curfew would apply at 22:00 for anyone under 13 years of age and at midnight for those under 18.
Flash mobs, organised through social networking websites, have left several city residents injured in recent weeks.
Mr Nutter said the curfew would go into effect on Friday evening.
The mayor said those who broke the curfew could be taken to a police station and fined up to $300 (£184), while repeat offenders faced penalties of $500.
'Biggest, baddest gang'
He warned that if parents failed to pick up their children, the department of human services would be contacted and parents could face child neglect charges.
A statement released by Mr Nutter's office said the number of police on patrol in the city would be increased until the start of the coming school year.
"I want to strongly encourage parents and guardians to be vigilant and to look out for their children this weekend," Mr Nutter said, announcing the plan on Monday.
"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," Mr Nutter added.
A stricter curfew will also go into effect on Fridays and Saturdays in several areas of the city, such as the historic downtown district, ordering youths under the age of 18 to be home by 21:00.
"What is making this unique today is the social media aspect," Everett Gillison, Philadelphia's deputy mayor for public safety, told the Associated Press news agency.
"They can communicate and congregate at a moment's notice," he added.
In the most recent incident, a flash mob of up to 40 youths launched an unprovoked attack on two men in Philadelphia's Center City district.
Several youths were arrested, including one as young as 11 years old.
A 23-year-old Philadelphia resident pleaded guilty in June to aggravated assault in the 2009 beating of a bicyclist in the popular South Street neighbourhood, an attack the city said was sparked by a flash mob.
Flash mobs emerged in 2003 as harmless public performances, but have taken on a darker twist in recent years.