A senior policeman has said young children were involved in rioting in north Belfast on Monday which he said was orchestrated by criminals
Assistant Chief Constable Alistair Finlay said his officers faced an "extraordinarily dangerous situation".
Twenty-one police officers were injured in the Ardoyne area and two blast bombs were thrown in violence after the passing of an Orange parade.
Mr Finlay said at least one gunman fired at police to try to murder them.
A BBC reporter saw a masked rioter fire a single shot from a handgun towards the police.
A loaded rifle was also handed into police after a group of children were seen playing with it.
Police said they fired 18 baton rounds during the trouble.
"It was dangerous, particularly from the sinister element of people amongst that crowd wanting to shoot police officers, wanting to kill police officers," Mr Finlay said.
"Wanting to derail whole processes through that, so it was an extraordinarily dangerous situation."
The Real IRA has been blamed by Sinn Fein for the trouble.
Ardoyne priest Father Gary Donegan said the trouble was started by outsiders.
"Myself and many people were looking at people last night that we'd never seen in the area before in our lives.
"It was as if people had been bussed into the area for this very purpose and that this was being very much orchestrated," he said.
Police promised a "rigorous investigation" to identify those who had taken part in the trouble.
Two more police officers were injured in other incidents across Northern Ireland.
In the early hours of Tuesday, police and Army bomb disposal experts were called to a security alert in Lurgan, County Armagh. It was sparked by a suspicious object in a car that was hijacked earlier. It was later declared a hoax.
There was a similar incident in Strabane, County Tyrone.
In the Memorial Hall area of Londonderry, 39 petrol bombs were thrown during Monday's disturbances, but there were no reports of any injuries or arrests.
Assistant Chief Constable Alistair Finlay: Rioting is a "sinister development"
Earlier rival groups had taunted each other as Orangemen and a small number of bands made their way through the city's Diamond area.
Both sides spat at each other and threw missiles. Police separated factions, but were themselves attacked by nationalists throwing stones and bottles.
In Armagh, police came under attack with missiles and paint following a security alert at Friary Road in which a minor explosion occurred.
Four people have been arrested for public order offences and at least two cars were also stolen and set alight in the city.
In Rasharkin, County Antrim, officers also sustained minor injuries when they were struck by stones and bricks. One man was arrested.
The Sinn Fein leader Gerry Adams condemned the violence and said the Orange Order should consider re-routing a small number of contentious parades, including those which pass the Ardoyne.
"Why play into the hands of those who orchestrated last night disturbances?," he said.
"I would appeal to the Orangemen - they're not giving a victory to anyone if they just take an alternative route to where they want to go."
He also called for the leadership of the Orange Order to talk to Sinn Fein.