Authorities are clamping down on teenagers terrorising the US city of Philadelphia.<br /><br />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.<br /><br />Seventy-five people have been arrested for violating the newly enforced curfew.<br /><br />Al Jazeera's John Terrett reports from Philadelphia.
