Sectarian tension in Belfast has spilled over into violence, leaving 23 police officers and an MP injured.
Democratic Unionist Party MP Nigel Dodds, who had been near the police lines talking to officers, was taken to hospital after being struck on the head by a brick during the fighting, a party spokesman said. His condition is said to be stable.
Officers were pelted with masonry, bottles and other missiles during clashes with loyalists in Woodvale Road.
The Police Federation for Northern Ireland said 23 officers had been injured in the north Belfast disturbances as they dealt with "serious public disorder".
Water cannon were deployed and around 20 non-lethal rounds known as Attenuated Energy Projectiles fired.
Officers were hit by an array of missiles including sticks and fireworksWater cannon were also used by police in east Belfast in response to missiles hurled by crowds of people.
Trouble erupted as the Orange Order protested against the decision to ban a contentious July 12 parade from a north Belfast flashpoint.
Loyalist bandsmen played sectarian tunes at police lines while bricks and bottles were thrown.
At one point a rioter broke down a section of wall and threw it at police shields. Others used sticks and pieces of drainpipe to batter police vehicles, while some danced on top of the force's Land Rovers and tried to rip off protective plating.
Police were enforcing a Parades Commission ban. The ruling stops Orange Order lodges from walking on a stretch of road in north Belfast that separates loyalist and nationalist communities.
Water cannon is used on loyalist protestersThe Parades Commission - the adjudicating body set up after the Good Friday Agreement peace deal - had banned marchers from a 300-metre stretch of the Crumlin Road past the nationalist Ardoyne area.
Three lodges from the Ligoniel area had been able to walk the contested route out, but were prevented from making the return journey.
Trouble also broke out on the Newtownards Road in east Belfast. Bricks and bottles were thrown in both areas.
One man was arrested for disorderly behaviour and another for provocative behaviour.
Serious rioting has erupted after previous Orange Order evening marches in Ardoyne. Last year shots were fired at police, who also came under sustained attack from petrol bombs thrown by republican protesters.
MP Nigel Dodds was taken to hospital after his head injuryBefore he was injured, Mr Dodds said: "This sort of situation here creating a bottleneck at this junction, the Parades Commission decision to block it here, this is the inevitable outcome.
"The police were well aware that was the danger the Parades Commission had put them in. I would appeal for everybody to stay calm, not to get involved in violence.
"The Grand Lodge have made it clear they do not want violence and the people who want to engage in violence should desist immediately."
Sinn Fein MLA Gerry Kelly blamed the violence on the Orange Order and unionist politicians.
"Speech after speech at the various (Orange) demonstrations were clearly designed to stir up sectarian tension and have alongside the Orange Order's failure to abide by Parades Commission determinations led directly to the violence in Belfast tonight.
"No amount of hand wringing or denial in the coming days from the Orange Order and unionist politicians can alter that reality.
Twenty-three officers were hurt during the 'serious disorder'"People had a right to expect better, instead what we got was a very deliberate strategy with the inevitable results being seen on the streets this evening."
The Orange Order has appealed for calm and asked that all parades be peaceful.
In a statement, it said: "The Parades Commission must go, but violence is not the way to achieve it. All protests must be peaceful.
"In support of the Grand Orange Lodge's appeal for calm, the Ligoniel Lodges (north Belfast) have decided to suspend their protest over the determination for the Crumlin Road."
DUP leader and Stormont First Minister Peter Robinson called for calm.
"Violence and attacks on the PSNI and the wider community are wrong, can never be justified and must stop," he said.