By Alex Richardson

BELFAST (Reuters) - A train carrying Protestant marchers to a parade has been pelted with petrol bombs as violence grips Northern Ireland's "marching season".

On Monday, 25 police officers were injured in sectarian clashes surrounding a bitterly contested "Orange" parade in Belfast.

"Seven petrol bombs and a paint bomb were thrown at the train, damaging a number of windows on it," said Superintendent Alan McCrum on Tuesday. "There are no reports of any injuries."

The marching season, in which Protestant organisations, principally the Orange Order, hold parades celebrating a battlefield victory in 1690 over Catholics, is traditionally a source of sectarian friction in the British-ruled province.

Tuesday's incident happened as the train pulled out of a station near a Catholic estate in Lurgan, southwest of Belfast. There had been minor outbreaks of trouble elsewhere overnight, with petrol bombs thrown at police in Antrim and Londonderry.

Hundreds of Protestants and Catholics pelted each other with stones and bottles in a flashpoint district of north Belfast after a contentious parade on Monday night.

Catholics later fought with riot police and troops, but on Tuesday nationalists blamed the authorities for the trouble, saying it could have been avoided if Protestant "loyalists" had not been allowed to follow the parade past a Catholic area.

"I have no apology to make in terms of how my officers behaved," Belfast's police chief, Assistant Chief Constable Duncan McCausland, told reporters on Tuesday.

"They clearly behaved extremely professionally, as did my army colleagues, in terms of how we policed the difficult situation we were in last night, but that situation was not of my making."

Monday night's clashes broke out after Orangemen marched past the Catholic Ardoyne neighbourhood in a parade celebrating Protestant William of Orange's victory over Catholic King James II at the Battle of the Boyne on July 12, 1690.

Protestants were angry the authorities had restricted the number of people who could march on the route while Catholics, who view the yearly parades as triumphalist and provocative, said the march should be banned altogether.

Hundreds of bystanders from both sides of the sectarian divide threw missiles at each other over the top of the parade and over a huge blockade of police and British Army vehicles and armoured cars set up to keep them apart.

Catholic protesters then fought running battles with troops and riot police, who responded briefly with water cannon.

"It was in many ways the same old story," said Gerry Adams, leader of the Irish Republican Army's political ally Sinn Fein. "Nationalists were herded into their areas, were hemmed in, were beaten when it came to it, and the loyalists were shepherded through."

Irish Prime Minister Bertie Ahern said in a statement on Tuesday the violence was "deeply regrettable and disturbing".