More than 20 police officers have been injured during serious rioting following the passing of a Protestant parade through a nationalist area of north Belfast.
Their injuries are not believed to be serious.
Petrol bombs and bricks were thrown at police by nationalists and loyalists in Ardoyne.
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.