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Authorities are clamping down on teenagers terrorising the US city of Philadelphia.

The mayor announced a 9pm curfew for youths under 18 after flash-mobs, often organised through social media sites, carried out a wave of random attacks.

Seventy-five people have been arrested for violating the newly enforced curfew.

Al Jazeera's John Terrett reports from Philadelphia.

Source: Al Jazeera