By Ian Graham
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 favor 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.
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.
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.
"We have a serious situation here," said Nigel Dodds, a lawmaker representing North Belfast at the London parliament. "Things are very, very tense across Belfast. We are working to calm things."
In some parts of the city, Protestants and Catholics rained missiles down on each other as well as at police.
(Editing by Sam Cage and Alison Williams)