Up to six shots were fired at police during a third night of violence in north Belfast on Tuesday.
Petrol bombs and a pipe bomb were also thrown by nationalist rioters in the Ardoyne area. About 100 officers used water cannon and baton rounds.
The prime minister has said the PSNI behaved in a brave and restrained way during the recent trouble.
Speaking in the Commons, Prime Minister David Cameron described the violence as "unacceptable" and said the PSNI had been forced to respond.
On Wednesday, Assistant Chief Constable Duncan McCausland said children as young as 10 were involved in the violence.
He said as well as throwing stones and petrol bombs, young children were being used as shields by "sinister elements" organising the riots behind them.
ACC McCausland said police had hours of video footage from before the violence started and would use it to identify the rioters.
"There will be significant arrests in the forthcoming days - individuals will not go scot free," he said.
The rioter who dropped a concrete block on a policewoman on Monday night - seriously injuring her - had already been identified he added.
"Wherever he is in Northern Ireland he can sit and be worried - we will be coming for him," he said.
The officer is due to get out of hospital later on Wednesday.
Meanwhile, the NI first and deputy first ministers have met the chief constable.
Following the meeting, Peter Robinson said he had "nothing but the highest admiration for the way the police has dealt with the difficulties of recent days".
Martin McGuinness described the violence as a "setback against the huge progress we have made in recent times".
But he insisted that those behind the violence "would not succeed" in disrupting the political process.
The NI Chief Constable, Matt Baggott, said his force was determined to bring those responsible to justice.
Mr Robinson and Mr McGuinness were criticised on Tuesday by Assistant Chief Constable Alistair Finlay, who called on them to provide more leadership.
On Wednesday, searches were carried out in the Ardoyne area in relation to the recent trouble and a number of items were taken away for further examination.
A 20-year-old man was also charged with riotous behaviour in connection with an incident during an Orange Order march in the Ormeau Road area on 12 July. He is due to appear at Belfast Magistrates Court on Thursday.
On Tuesday night in Ardoyne, burning barricades were put in place and laser pens were shone at police. Some officers suffered minor injuries.
Politicians have condemned the violence, which has flared at the height of the loyalist marching season.
Sinn Fein have said dissident republicans and "anti-social elements" are behind the violence.
SDLP councillor Nicola Mallon spent most of Tuesday night in Ardoyne and said it as a night of "absolute madness and mayhem" with local residents growing increasingly worried about the situation.
Northern Ireland's Justice Minister David Ford said there was an "extremely sinister edge", in terms of those who were encouraging the rioting.
There was also trouble in North Queen Street area of north Belfast on Tuesday night where youths built a barricade across the road.
In the Markets area of south Belfast a bus was damaged by stone-throwers and a car was recovered by police as youths tried to hijack it.
A car was also stolen in Lurgan, County Armagh and four petrol bombs were thrown at a police vehicle in Londonderry.
More than 80 police officers had been injured over the previous two nights of rioting by nationalist youths.
Police later released aerial footage of the violence, which showed officers coming under sustained attack from people throwing bricks and wielding metal bars and planks.
The chief constable, Matt Baggott, said the cost of policing Monday's violence would run into millions of pounds.
A senior police officer has said there will be "significant arrests" of those involved in rioting in Belfast.