WARSAW  - Riot police in the Polish capital used truncheons to try to break up a crowd of right-wing extremists who were pelting them with firecrackers and lumps of concrete. The violence broke out at the start of a parade to celebrate Poland’s national holiday, which in the past has proved a catalyst for clashes between rival extremist groups. The right-wing protesters were preparing to start their march in Warsaw when some threw firecrackers and projectiles at police in riot gear.