Three police officers have been shot during violence in north Belfast on the eve of the annual Twelfth parades.
The officers, one a woman, were injured by a masked man firing a shotgun at North Queen Street. One male officer remains in hospital with an arm wound.
A total of 27 officers were injured during disturbances - 14 in the New Lodge area of north Belfast and 13 at Broadway in west Belfast.
About 200 people threw petrol bombs, stones and bottles at Broadway.
The trouble began at about 2345 BST on Sunday. Police used water canon and fired baton rounds at the nationalist rioters.
The police ombudsman is investigating the incident which is routine procedure when police fire baton rounds.
During the rioting, at least one car was hijacked and set alight.
It happened as police formed lines to separate those in the nationalist Broadway area from people attending traditional loyalist eleventh night bonfires on the nearby Donegall Road.
The trouble in the New Lodge in north Belfast was also described as "sustained".
There was also sustained rioting in the Broadway area last weekend.
The police said that trouble had been "orchestrated" with Sinn Fein blaming dissident republicans.
Chief Superintendent Mark Hamilton said the officer who remained in hospital with the gunshot wound was not seriously injured.
"But clearly he is also trying to come to terms with the fact that someone tried to murder him last night," he said.
Chief Superintendent Hamilton said that in both north and west Belfast members of the public and community representatives tried to help police stop the violence.
However, he said there were small elements "who seem hell-bent on wrecking their own communities".
"What last night has shown is that disorder does nothing but damage local communities," he added.
"Today, we don't want to see any more communities damaged, any more cars hijacked, any more property damaged and any more police officers injured in the course of their duties."
NI Justice Minister David Ford said the violence must not be allowed to undermine political progress.
"Clearly, there are small numbers of people from republican backgrounds who do not accept the settlement which the vast majority of the rest of this community has accepted," he said.
"There is an element where this is then being translated into acts of violence on the ground - whether at the full paramilitary level that we have seen or in the street violence we have seen in Belfast last night and a few nights earlier."
DUP south Belfast MLA and policing board member Jimmy Spratt said it was "deeply worrying" that a gun was fired during the rioting.
"Whilst all rioting is evil, this was a direct attempt to murder police officers and should be condemned right across the community," he said.
He appealed to those taking part in Twelfth parades to remain "calm, dignified and vigilant".
SDLP west Belfast MLA Alex Attwood also condemned the rioting.
"The community will be resolute in its opposition to the small elements involved," he said.
"This must not continue or escalate. Over the next crucial hours, people should be strong and calm and not respond to provocations."
The 'Twelfth' is the annual high-point of the Protestant Order's parading calendar.
The marches commemorate Prince William of Orange's 1690 Battle of the Boyne victory over Catholic King James II.