Police have accused republican paramilitaries of orchestrating serious disturbances in east Belfast which left 13 officers injured over the weekend.
Two PSNI officers remained in hospital last night, one of them with head and spinal injuries, after they came under attack in the nationalist Short Strand enclave on Saturday night in what a PSNI spokesman described as a "sustained and orchestrated attack".
Petrol-bombs and breeze blocks were thrown at the security forces who in turn fired one plastic bullet.
While not prepared to speculate which organisation was behind the riots, Supt Graham Shield said the "degree of intent and ability would have taken people with good organisational skills.
"Our officers were subjected to a vicious assault and everything was done to obstruct efforts to restore normality," he added.
The clashes began after members of the loyalist Apprentice Boys returning from a major parade in Derry left coaches on the Albertbridge Road.
According to police, a nationalist crowd of at least 200 had gathered nearby hurling missiles across the peace-line at homes in the loyalist Cluan Place area. A number of vehicles were reportedly used to form a barrier to stop police and British troops from intervening.
"When police attempted to restore order they were prevented from entering the [nationalist] Clandeboye area by an organised protest involving women and children who were sitting across the street," the spokesman added.
Nationalists in the Short Strand yesterday rejected the police claims, saying it had been their houses which had come under stone-throwing attacks from loyalists first.
The Sinn Féin councillor for the area, Mr Joe O'Donnell, blamed the Apprentice Boys for the events, saying he would submit photographic evidence to the North's Parades Commission.
"Upon the dispersal of an Apprentice Boys parade a large number of people including band members were allowed to enter Cluan Place where they launched a full-scale attack on residents including children taking part in a summer festival," he claimed.
A former Ulster Unionist lord mayor of Belfast, Mr Jim Rodgers, however, accused republicans of orchestrating the violence.
"Once again we have the republican movement deliberately putting out misleading statements in order to put the blame elsewhere. I totally condemn the disgraceful but well-organised attacks on the police and challenge Sinn Féin/IRA to do likewise," he added.
In Derry, a 15-year-old Catholic boy sustained serious facial and head injuries after being set upon by up to nine young people, male and female, when walking home from a friend's house in the Church Brae area on Saturday night. The teenager was struck in the face with a bottle and was then repeatedly kicked and punched as he lay on the ground. One of the assailants was seen kicking him in the head.
A PSNI officer, Insp Bob Torrens, said detectives were treating the attack as sectarian. Given its viciousness, some of the attackers were likely to be spattered with blood, he added.
"I understand he [the victim] was wearing a Republic of Ireland football shirt and may have attracted attention in that way but it was quite a brutal assault. He was badly beaten and has a very bad cut to the head, a badly bruised eye and a suspected fractured nose."
It has emerged that British government officials will hold talks with loyalist paramilitary representatives this week in an effort to stop sectarian violence. Speaking on the BBC's Inside Politics programme, a Northern Ireland Office minister, Mr Des Browne, said it was important to keep open such a channel of communication.
"I am going into a sequence of meetings this week with organisations including trade unions, the civic forum and indeed those people who put themselves forward as representing organisations who appear to be involved in the violence," he stated.