BELFAST  – Northern Ireland police fired water cannons and plastic bullets Monday to disperse youths throwing petrol bombs as the Protestant marching season got off to its most violent start in years.

At least 10 police officers were injured as violence erupted in a flashpoint area of northern Belfast and several other towns and cities, said a spokesman for the police force in the British province, the Royal Ulster Constabulary.

Three of the police injuries happened in the northern town of Armagh, where a small bomb also exploded close to the route of the marchers.

"This was an extremely disturbing development. The bomb was clearly left there either to kill or cause maximum disruption," an Orange Order spokesman said, adding that the parade was not affected.

Protestants from the Orange Order hold marches across Northern Ireland every year, an annual tradition that typically heightens tensions between them and the Catholic community.

The marches commemorate the victory of Protestant King William of Orange over James II's Catholics in Ireland in 1690.

Several of the parades have become flashpoints over the years because they pass close to Catholic neighbourhoods -- one hotspot in particular being around the Catholic enclave of Ardoyne in the capital Belfast.

"Petrol bombs, paint, fireworks, stones and bottles have been thrown at police in the Ardoyne area," the Royal Ulster Constabulary spokesman said.

In further violence in Belfast rioters fired a gun shot at police and hijacked a van which was pushed at police lines, the spokesman said.

"Water cannon have been deployed in the area and at least 14 AEPs (plastic bullets) have been discharged by police," he said.

Republican politician Gerry Kelly blamed the violence in north Belfast on the Real IRA, a dissident republican splinter group opposed to the peace processs in Northern Ireland.

"The Real IRA or whatever they may call themselves and some other splinter organisations sent people over here with the sole aim to cause riots, to bring this further down into sectarianism," he told the BBC.

Six officers were injured in Belfast during the violence, and another in the northwestern city of Londonderry where unrest also flared.

Police also came under attack from missile-throwing youths in the village of Rasharkin in northeast County Antrim.

A 1998 peace accord ended most of the violence between Catholic republican and pro-British Protestant groups, which had plagued Northern Ireland for three decades, killing at least 3,500 people.

Devolved self-rule is now in place in the province after a landmark accord in 2007 between the Protestant Democratic Unionists (DUP) and Catholic Sinn Fein.

Despite the peace accord, two British soldiers and a police officer were shot dead within 48 hours of each other in March, killings blamed on dissident republican paramilitaries.

They were the first such killings in roughly a decade and raised fears of a return to civil unrest.

However political parties from all sides in Northern Ireland have spoken out against the shootings, while thousands of people joined peace rallies across the province showing solidarity against the attacks.