It's August here in the northeastern city of Philadelphia and nearly 50 teenagers have just been rounded up by police for breaking a 9pm Friday night curfew, imposed to combat a spate of flash mob attacks.
Flash mobs can be quite effective when multiple people turn up in one place to attract attention to a just cause by simultaneously wearing comic clothing or mass break-dancing.
They can, however, be terrifying when they're violent and unnecessary, as we have seen in "the city of brotherly love", as Philly likes to be known.
Marauding teenagers have been damaging street furniture, dragging at least one man from his vehicle, punching innocent passers-by and robbing shops, all in the blink-of-an-eye, before moving on to be destructive somewhere else nearby.
Police say the gangs use social media to co-ordinate their activities.
But most seriously for a tourist town like Philadelphia is that tourists themselves have been harassed - and no city can afford that.
The mayor, Michael Nutter, has had enough.
"This nonsense must stop. You want to act like a butthead, your butt is going to get locked up."
Mayor Nutter blamed the violence on what he called a small number of knuckleheads and brought in the 9pm curfew for anyone under 18.
Scores of teen were hauled off the streets on the first night. Their parents were forced to pick them up from police stations.
I couldn't find anyone on the streets of Philadelphia who didn't think the curfew was a good idea - though many seemed a little afraid to talk about it on camera.
No I don't think the curfew's unfair, I think if the children, if they can't behave themselves in Philadelphia then they need to be in the house at nine o' clock."
The curfew's fine, the fine might be a little steep but you know the authorities are looking to get a message over to the parents. .. keep watching your kids."
It's gonna make it better because the people who might want to start it are the 17-and-under crowd, that are allowed to be out 'til midnight."
Several civil liberties and youth rights organisations are on record as saying they think Philly's gone a bit far. But one man told me he thought the National Guard would be called out if there was any serious attempt to replicate what's been happening in London recently.
While we were in Philadelphia, a group of teenagers involved in flash mobs had just turned themselves into police. Their school administrator had identified them from city centre security camera video. Lt Patrick Doherty, from the Philadelphia Police department, said:
There's a lot of cameras in city centre, so if you've gonna do something in city centre, you can count on someone picking it up or seeing it."
The teens get a warning for a first offence but fines could run as high as $500 if they do it again and get caught.
Philadelphia is extending recreation centre and library hours to give kids somewhere to go during the long summer vacation. The mayor says the curfew will stay in place 'til the schools go back ... and can be extended if needs be.