By Ian Graham

BELFAST (Reuters) - Northern Ireland is drafting in more than 600 police from mainland Britain during marches planned this week by the province's Protestants, a potential flashpoint for sectarian violence, the local police chief said on Tuesday.

Pro-British Protestants march every summer in the British-ruled province, a tradition seen as provocative by Irish nationalists and which regularly leads to violence. There will be more parades than usual this year - about 550 - because of an anniversary.

Usually the province relies on local police but this year is drafting in unarmed officers who helped protect last month's G8 summit, Police Service of Northern Ireland Chief Constable Matt Baggott said.

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.

Tens of thousands of members of the Orange Order will parade on Friday 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.

The 100th anniversary of the signing of a covenant by Protestants against greater autonomy for Ireland, means there will be more marches, creating "unique demands" and a need for more security personnel, Baggott said.

"2013 is a year when there is a real opportunity to break the cycle of violence," he told reporters.

A police spokeswoman said she was not aware of a previous occasion in which police had been drafted in for the parades, though the current force is much smaller now than during the years of violence known as the "Troubles".

(Editing by Sam Cage and Robin Pomeroy)