BELFAST (REUTERS) - Protestant marchers in Belfast threw bricks and bottles at police, who responded with water cannon and rubber bullets as Northern Ireland's annual parade season descended into violence on Friday.
Pro-British Protestants march every summer in the British-ruled province, a regular flashpoint for sectarian violence as Catholics, many of whom favour unification with Ireland, see the parades as provocative.
Since a peace agreement was signed in 1998, violence between Catholics and Protestants - which raged on and off for three decades - has largely ended. But much of Belfast remains divided along religious lines and unrest still flares from time to time.
Extra police have been drafted in from Britain for the marches.
Mr Nigel Dodds, a lawmaker representing North Belfast at the London parliament, was hit on the head by a missile and taken to hospital.
Extra police were also sent in for last month's G8 conference, which passed without serious disorder.
"The international image of Northern Ireland post-G8 has been seriously damaged by the scenes of rioting," said Mr David Ford, Northern Ireland's justice minister.
Tens of thousands of Orange Order marchers, wearing orange sashes and waving British flags, paraded at more than a dozen venues across Northern Ireland to mark the 1690 victory at the Battle of the Boyne by Protestant King William of Orange over Catholic King James of England.
Police said four officers had been injured in the clashes, on the borders of Protestant and Catholic areas.
In some parts of the city, Protestants and Catholics rained missiles down on each other as well as at police.
The Orange Order is upset this year because authorities ruled they could not walk along a stretch of road that divides the two communities, a flashpoint that has regularly sparked trouble in the past.
"Those who are using the cover of protest to attack the police are massively damaging the cause they support," said Northern Ireland's first minister, Mr Peter Robinson, who heads the devolved government.
"Violence is undermining a just cause and runs totally against the wishes of the Orange Order for protest to be entirely peaceful."