Media playback is unsupported on your device Media caption Crowds have been seen beating and robbing people

Philadelphia's authorities have ordered a weekend evening curfew for under-18 years old in an effort to deter teenagers from wreaking havoc in so-called flash mobs.

As Britain looks to America for answers to the rioting that has rocked England over the last week, the BBC's Laura Trevelyan visits Philadelphia to see how its new measures will work.

The phenomenon known as flash mobbing, where groups of teenagers gather after alerting one another via mobile phone, Twitter and Facebook and rampage through the town centre, has frightened residents and alarmed Philadelphia's city government.

While the scenes haven't been as violent or as sustained as those in urban England in recent days, crowds of teenagers have beaten and robbed people and shops have been damaged.

Last Friday 7 August saw yet more trouble in the City of Brotherly Love.

'You've damaged your race'

Emily Guendelsberger, a magazine editor, was knocked over by a mob of teenagers in June, just a few blocks away from her home.

Image caption Emily Guendelsberger was injured in a June flash mob

Her leg was so badly broken her knee is held together by screws.

"We were going to a friend's house at 21:30 on a Saturday night," Ms Guendelsberger said.

"The front line of the teenagers was yelling at us. Someone grabbed my bag. Someone punched me in the face. I fell over. My friend tried to get me to stand up and it wasn't happening."

Philadelphia Mayor Michael Nutter has vowed to put a stop to the violence, which risks undermining the city's attractiveness to tourists and restaurant-goers.

This is where the US Declaration of Independence was signed and the constitution debated - history which pulls in the visitors.

In a blunt address from the pulpit of his church last Sunday, the African-American mayor told the trouble-making teenagers: "You've damaged yourself; you've damaged another person; you've damaged your peers; and quite frankly you've damaged your race."

From Friday night, police officers can send children found on the city centre streets home once the clock strikes 21:00.

Blame the parents

An existing curfew for children has been made even earlier, and parents whose children repeatedly ignore the order could be fined up to $500 (£307).

Image caption Deputy Police Commissioner Kevin Bethel says he is praying for Britain's police

Philadelphia will keep some recreation centres open until 22:00 at the weekends so that the children barred from the city centre will have something to do.

At a basketball court in South Philadelphia, I asked a group of teenagers what they thought about the curfew and the flash mobbing which led to it.

Keri Scott, 16, and his friend Marcus were worried about the youths displaced by the curfew who might now come and play on their basketball court.

"That means now we're going to have bad people coming to a good court, and it's not cool," said Keri, who hopes to try out for a college basketball team.

Ben, 16, welcomed the curfew as a way to create a safe environment. He can't see the point of flash mobbing.

'Nothing to lose'

Although Jennifer, 15, has never been in a flash mob she sees it as a way for teens to express their anger and frustration and to force adults to listen to them.

Image caption Ben, 16, hopes the curfew will make the city safer

She says the curfew will make teenagers more "mad".

In Philadelphia's city centre, 25 police officers will patrol on bicycles on weekend nights. The bicycles can be used to form a barricade against a flash mob.

Kevin Bethel, the deputy commissioner of Philadelphia's police department, said police have asked popular DJs to spread the message about the curfew.

Deputy Commissioner Bethel says his prayers are with Britain's police as he sees what they are dealing with.

The challenge to policing on both sides of the Atlantic is how to manage groups that "can organise and move so quickly", he said.

Philadelphia's deputy mayor for public safety, Everett Gillison, also stressed the importance of firmness and engagement. He hoped the curfew would encourage parents to become stricter with their children.

"We need parents to say, 'the city is not your parent, I'm your parent. I need you to be home'," he said.

As Ms Guendelsberger learns to move about on her crutches, she wonders how effective the curfew can be in the end.

"I sound like I'm apologising for these kids and I'm not," she said.

"What they did was horrible. But we should at least explore why they feel they've got nothing to lose."