Syria's security forces are crushing of dissent town by town and conducting a mass round-up of opposition leaders, rights activists said Thursday. Thousands of students defied the crackdown to stage a protest in Syria's second-largest city Aleppo late Wednesday before being dispersed by loyalists and security force personnel, an activist said. Images from demonstrations in Aleppo and Barzeh, north Damascus, taken from YouTube. AFP can verify they were shot in Aleppo and Barzeh - they were presented on YouTube as being filmed on May 11, 2011.