The disturbances come at the height of Northern Ireland's marching season, a traditional flashpoint in the troubled province's history.

Unrest often flares as Protestant marchers, in favour of continued British rule of the province, pass through areas mainly populated by Catholics, who are generally opposed to rule from London.

'Outright thuggery'

In the most violent riots of the past few days on Monday, dissident republicans threw petrol bombs and concrete slabs at Protestant Orangemen and their police escort as they marched through Ardoyne.

Peter Robinson, Northern Ireland's first minister, and his deputy Martin McGuinness, who have appealed for calm, are to meet Northern Ireland's police chief later on Wednesday to discuss the ongoing tensions.

Both Robinson and McGuinness criticised the violence on Tuesday night, saying it was out of keeping with modern-day Northern Ireland.

"I am disgusted at the outright thuggery and vandalism that has taken place over the course of the last 48 hours," Robinson said.

"There is no excuse and no place for violence in civilised society ... We must keep our entire focus on defeating those who would seek through violence and destruction to drag us back."

Through dialogue

For his part, McGuinness said: "Our experience demonstrates that the way to deal with any disputes or contention is through dialogue and agreement."

Authorities are blaming a small group of troublemakers for the unrest.

Witnesses have also described children becoming involved.

"I was directly confronted by a nine-year-old last night," Father Gary Donegan, a local priest, told BBC radio, saying he had "physically pulled stones out of children's hands".

Despite the relative calm in Northern Ireland since the 1998 peace accords, violence frequently breaks out around July 12, which sees Protestants marking Prince William of Orange's victory over the Catholic King James II at the Battle of the Boyne in 1690.