A night of serious rioting after rival Protestant and Catholic parades were allowed to go ahead within hours of each other in the nationalist Ardoyne district of north Belfast has left 20 police officers injured.
The chairman of the Northern Ireland Parades Commission has defended the decision to allow to parades to take place.
The Commission was established to adjudicate on contentious marches, and even though there has been major trouble at this particular Catholic/Protestant flashpoint every year for over a decade, the chairman Peter Osborne said given the circumstances, the rulings were correct.
He also hit out at people whom he claimed stood on the sidelines criticising their work. The language and comments in the days leading up to the parades had heightened tensions and were not helpful.
Mr Osborne told BBC Radio Ulster: "It is complete and utter nonsense to blame the Parades Commission for the violence last night. There has been violence in this location for many, many years now."
He added: "It is time for politicians to take ownership of contentious parades... that's the way forward."
In Londonderry, Northern Ireland's second largest city, petrol bombs were thrown in the nationalist Bogside but no injuries reported.
One car has been set alight after a petrol bomb was thrown at it in the Fahan Street area.
Petrol bombs and bricks were thrown at police by nationalists and loyalists in Ardoyne. The officers' injuries are not believed to be serious.
Police responded by using water cannons to contain the crowds. Six non-lethal baton rounds have been fired by the security forces. There have been two arrests, although police expect more to follow.
Three cars have also been hijacked and two of them pushed at police. At least one was set alight.
Police faced "significant disorder" in nationalist parts of Ardoyne with around 10 gunshots fired at police lines in Brompton Park, a spokeswoman said. No officers were wounded.
A short distance away, officers were attacked on two sides with bricks and bottles hurled by loyalists.
The senior police officer in charge of the security operation in north Belfast, Assistant Chief Constable Will Kerr, urged individuals and communities affected to respond in a calm and responsible manner.
"Violence has serious and unwanted consequences for us all and we will robustly investigate all incidents of disorder," he warned.
There was also violence in Londonderry's nationalist Bogside, where petrol bombs were thrown at police and a car set alight. In south and east Belfast there were five arrests for a variety of offences including disorderly behaviour.
It followed a day of largely peaceful parades by the Orange Order across Northern Ireland. The Twelfth of July represents the culmination of the Protestant Orange Order's marching season. It has traditionally polarised Catholics and Protestants but violence has been confined to isolated pockets.
There has been trouble annually at Ardoyne in recent years following Orange parades through the area.
This year around 15 Orangemen with banners paraded in silence through the residential area of red brick terraced housing in a token demonstration. There was a separate parade by republicans as well as protests by nationalist residents and loyalists in the area.
Six men, aged 34, 28, 21, 19, 19 and 18, are to to appear at Belfast Magistrates' Court later today charged with various public disorder offences.
They were arrested following trouble in the Broadway area of the city which erupted on Wednesday night and lasted until the early hours of Thursday.
Another man, aged 23, will appear at the court on August 8 charged with assaulting and obstructing police.