Northern Ireland police say 47 officers have been injured during overnight clashes with Protestant extremists angry over a Belfast parade by Irish republicans from the Catholic side of the community.
Sunday night's violence came as Irish republicans paraded near British Protestant districts of north Belfast. Such demonstrations, usually by the Protestant side, trigger riots every summer in Northern Ireland.
Police donning flame-retardent suits, helmets and shields spent nine hours standing their ground versus mobs of masked, hooded Protestants. More than 100 rioters threw bottles, bricks, fuel bombs and fireworks at police lines until early Monday.
Police used a water cannon to disperse rioters.
Police commander Matt Baggott says his officers demonstrated "courage and restraint."
Chief Supt George Clark, the Police Service of Northern Ireland officer in charge of the operation, said in a statement on Monday: "I am both angry and sad that my officers have been subjected to such significant attack."
Most of those hurt suffered cuts and bruises. Four officers were hospitalized with more serious injuries.
One suspected rioter, a 17-year-old boy, was arrested Monday morning.