Several police have been injured and a civilian shot during loyalist rioting over the re-routing of a controversial Orange Order parade in Belfast.
Northern Ireland police chief Sir Hugh Orde said officers were attacked with explosives and shot at.
He said the Orange Order must bear "substantial responsibility" for the rioting over the Whiterock parade.
The clashes continued into early Sunday morning, and disturbances broke out in several towns in County Antrim.
Police said cars were hijacked and roads blocked in Ballyclare, Glengormley, Rathcoole, Larne and Carrickfergus.
DUP leader Ian Paisley blamed the Parades Commission for not reviewing the route that barred it from a nationalist area.
The parade was re-routed to avoid the mainly nationalist Springfield Road area.
After a request by unionists on Friday, the Parades Commission reviewed its ruling on the route, but decided not to change it.
"The commission treated elected representatives with contempt by its refusal to even call us to put our case," said Mr Paisley.
He also urged an end to the violence, which was continuing on Saturday evening in loyalist parts of Belfast and County Antrim with many roads blocked by protesters or burning vehicles.
He added: "At this difficult time, I am appealing to all law abiding people to remain calm."
The security forces came under sustained attack by several hundred rioters - some covering their faces with scarves and hoods - in the west and north of the city.
Tension had been building over the march route
Water cannon and plastic bullets were used against petrol bombers who attacked police and soldiers. Blast bombs and pipe bombs were also used, and automatic gunfire could be heard in several parts of the city.
At least six officers have been injured.
Cars were hijacked and set on fire on Ardoyne Road and North Queen Street.
'Shocking and depressing'
Protests also caused severe traffic disruption in the city. Several roads were blocked because, said one DUP councillor, there was a feeling of "disgust" over the parade route.
Almost 100 people blocked off three lanes of traffic behind Belfast City Hall.
Another group of protesters tried to block the Albert Bridge in east Belfast and were attacked by residents in the nationalist Short Strand.
Orangemen were infuriated by the decision to re-route the parade
BBC Ireland correspondent Kevin Connolly said while superficially such disputes seemed small they had come to symbolise for the Protestant and unionist community how they felt the peace process worked against them.
But he said people would find the level of violence "shocking and depressing".
"It's seven years since the Good Friday Agreement was signed. Today we had an echo of what Belfast sounded and felt like 30 years ago".
In terms of furthering political negotiation in Northern Ireland, the day's events "could hardly be more unpromising", he added.
Earlier, a number of children were left badly shocked after a bus they were travelling in was hit with bottles and stones.
A window was smashed and one passenger said some people on board panicked and were screaming in terror.
Mr Paisley and UUP counterpart Sir Reg Empey had been expected to address a rally of orangemen and their supporters at Woodvale Park following the parade but it was called off because of the trouble.
Orangeman Raymond Speers said: "In the grand scale of things, just to disrupt traffic is not a heinous crime when you look back over the years of history in Northern Ireland," he said.
"It's frustration of Protestant people as to what they can do to have their ordinary voice heard. We just feel so frustrated that there is a cultural veto through the Parades Commission for the republican/nationalist community."
Sinn Fein councillor Fra McCann said the trouble could have been avoided if the Orangemen had talked to Springfield Road residents.