Rioters attack police on historic Protestant cemetery . At least four police injured in two different locations , including three hit by shotgun blast . Police throw stones and explosives . Because of the sectarian nature of the holiday , celebrations have sometimes been marred by violence in the past . ` Disruption occurs on eve of holiday .
Rioters attack police on historic Protestant cemetery . At least four police injured in two different locations , including three hit by shotgun blast . Police throw stones and explosives . Disruption occurs on eve of holiday .