A missile knocked a leading Northern Irish politician unconscious as sectarian tensions escalated into rioting in Belfast at the climax of the Protestant marching season.
Nigel Dodds, the deputy leader of the Democratic Unionist Party - the province's pro-British, Protestant, conservative largest party - was taken to hospital after being hit in the clashes between Protestant loyalists and riot police.
It followed a day of what had been peaceful Twelfth of July parades throughout Northern Ireland.
Rioting Protestants, some wielding swords, attacked police at a roadblock across the loyalist marchers' preferred route through north Belfast.
Petrol bombs, sticks, fireworks, bricks, bottles, beer cans, drain pipes and part of a wall were hurled at police.
Teenagers were among the rioters, who attacked police vehicles and officers in riot gear.
Loyalist bandsmen played sectarian tunes.
Police fired about 20 plastic bullets and used water cannon.
Seven officers were injured, at least three of them knocked unconscious.
Dodds has represented North Belfast in the British parliament since 2001. He had been urging people to desist from violence at the roadblock.
The July 12 parade is the culmination of the Orange Order's marching season, and is usually accompanied by violence. The day is a public holiday in Northern Ireland.
The march marks the Protestant King William III's victory at the Battle of the Boyne in 1690 over the deposed Catholic King James II.
The marchers like to stick to traditional routes but there are often clashes as they sometimes pass through what have now become Catholic areas.
The violence broke out following a decision to bar loyalists from walking through a contested flashpoint area where Catholic republicans have gathered in the past to attack police.
About 3500 people died in the three decades of sectarian violence between Northern Ireland's Catholics and Protestants that largely ended with a 1998 peace agreement, though sporadic unrest and bomb threats continue.