Rioters hurled 130 petrol bombs at the RUC in Londonderry injuring at least eight officers in violent protests against the loyalist Apprentice Boys march.
On Belfast's Ormeau Road, police in riot gear were involved in ugly clashes with Catholics, who staged a sitdown protest. Each side blamed the other for the violence, while the Irish government later criticised the RUC.
As the day wore on there were more clashes between nationalist youths and police in Londonderry before and during the loyalist Apprentice Boys of Derry parades to mark the lifting of the siege of the city in 1689, the commemoration of which is one of the most important days of the Protestant marching season.
Nineteen police and a number of civilians were injured, none seriously, as youths threw stones, bricks, golf balls, paint and petrol bombs.
In Lurgan, Co Armagh, five men were arrested after a mob pelted police lines with petrol bombs, stones and bottles. Forty miles away, shops in Omagh, Co Tyrone, closed for an hour as people marked the first anniversary of the bombing which killed 29.
The Irish government said it was disturbed by the violent scenes, and junior minister Chris Flood said he did not consider the RUC had exercised restraint in dealing with the Ormeau Road protest.
He added: "We have heard first-hand and very detailed reports and a local priest has expressed great anger to the Parades Commission. I am wondering whether it was right in this case to allow the parade to go ahead."
The priest, Father Anthony Curran, accused police of being heavy-handed. He also said he had approached the chairman of the Parades Commission, Alistair Graham, and told him he should be ashamed of himself for permitting the march.
The Belfast residents reacted to the march with a makeshift barricade across the Ormeau Road, strengthened by a sit in. When police in full riot gear arrived there were angry scenes. Some protesters kicked and punched officers while policemen freely used their batons.
Residents' spokesman Gerard Rice claimed: "The police were brutal. It was the worst I've ever seen, I'm relieved no one in my community is dead. I have never seen brutality like I saw from the RUC today."
But RUC Assistant Chief Constable Bill Stewart rejected the criticism as "outrageous", maintaining that a minimum amount of force had been used.
He added: "While many people did behave non-violently, others reacted with disgraceful violence in obstructing police officers charged with carrying out a legally binding ruling from the Parades Commission."
The smaller Belfast parade went ahead once the road had been cleared. Its participants then then joined 10,000 others in Londonderry for the main event.
The Government last night condemned the violence. Northern Ireland Office minister Adam Ingram said the trouble arose because some people had violently challenged the lawful Parades Commission decision.
"Anyone who feels aggrieved by its decisions can challenge them through the courts, or stage lawful and dignified protest. But to take unlawful and violent action is unacceptable to a democratic society."
Apprentice Boys leader Alistair Simpson said: "Our parade went very well. Everyone did as they were instructed and they did not break ranks."Reuse content