Police officers firing plastic bullets and powerful water cannons clashed Tuesday with Roman Catholic protesters in Ardoyne, an Irish nationalist enclave in northern Belfast, as the annual day of Protestant marches reached a tense climax. Hundreds of Catholic youths lashed out at the police both before and after the marches by the Orange Order, a Protestant brotherhood whose July 12 demonstrations celebrate 17th-century military triumphs over Catholics — and often inspire a violent response. Earlier, 24 police officers and an unknown number of rioters were reported injured.