Undercover Israeli police officers detain a Palestinian suspected of throwing stones during clashes in the East Jerusalem neighbourhood of Issawiya May 15, 2011. Israeli security forces had been on alert for violence on Sunday, the day Palestinians mark the "Nakba", or catastrophe, to commemorate the expulsion or fleeing of some 700, 000 Palestinians from their homes in the war that led to the founding of Israel in 1948. REUTERS/Baz Ratner (JERUSALEM - Tags: POLITICS CIVIL UNREST ANNIVERSARY)