Rioting in Belfast on Wednesday night was on a scale not seen in Northern Ireland for years, police have said.
During several hours of violence, police officers were attacked, petrol bombs were thrown and a bus was burnt.
Eight officers were injured at an interface between loyalist and nationalist areas in west Belfast.
Senior police sources now say there was no evidence of UVF organisational involvement in last night's trouble in west Belfast.
Earlier the Police Service of Northern Ireland (PSNI) had suspected that individuals within the paramilitary group had encouraged some of the trouble.
However, following the progress of an investigation, police are now satisfied the group and its leadership was not involved.
Northern Ireland's power-sharing executive said it was "gravely concerned" by recent street violence and has called for calm to be restored.
Prime Minister Boris Johnson has also condemned the disorder.
Disorder on previous occasions - in south Belfast, Newtownabbey and Londonderry - has been connected to the Ulster Defence Association (UDA).
Earlier, PSNI Assistant Chief Constable Jonathan Roberts said it was likely that paramilitary organisations were involved and had planned the rioting on Wednesday night.
"Last night was at a scale we haven't seen in Belfast or further afield in Northern Ireland for a number of years," he said.
"We are very, very lucky no-one was seriously injured or killed last night given in particular the large number of petrol bombs thrown."
Police officers were called in from other parts of Northern Ireland to help to deal with the rioting.
More than 50 officers have been hurt in violence in several areas since the end of last month.
The bus driver whose double-decker was attacked and burned is "very shaken by the incident but is physically unhurt", according to the public transport provider Translink.
The PSNI is concerned at the potential for further trouble in the days ahead.
According to police, Wednesday night's trouble involved "equally large numbers on both sides" of the peaceline at Lanark Way.
It is believed the PSNI will be paying close attention to interface areas and the risk of further clashes between communities.
The activities of dissident Republicans are monitored as routine and the police are wary of any attempt to escalate the situation.
Assistant Chief Constable Jonathan Roberts was asked at a press conference if he feared weapons could come on to the streets.
"Given the history of Northern Ireland, it is something always in the back of our minds," he said.
Meanwhile, a group linked to the south Belfast UDA has issued a statement urging calm.
"The way forward must be through politics alone," the message from the Ulster Political Research Group read.
"Rioting, street disturbances and the destruction of property will not change what must be solved by our political representatives."
It cited the Irish Sea border and policing as the main problems.
The group is associated with the Jackie McDonald, the man widely regarded as the UDA leader in south Belfast.
The Loyalist Communities Council, an umbrella body which includes representatives of the UDA and UVF, was due to meet on Thursday.
It has not issued any public statements since it published a letter it sent to Boris Johnson in March warning of the danger created by the Irish Sea Border.
Some unionists have linked the violence in recent days to the decision by the Public Prosecution Service (PPS) not to prosecute anyone who attended the funeral of former IRA man Bobby Storey in June 2020.
It was attended by 2,000 mourners - including Deputy First Minister Michelle O'Neill - at a time when Covid-19 restrictions were in place.
They have also linked it to simmering loyalist tensions over the Irish Sea border imposed as a result of the UK-EU Brexit deal.
In recent days 10 people have been arrested as a result of rioting by gangs of people, some as young as 13.
Prime Minister Boris Johnson said: "The way to resolve differences is through dialogue, not violence or criminality."
I am deeply concerned by the scenes of violence in Northern Ireland, especially attacks on PSNI who are protecting the public and businesses, attacks on a bus driver and the assault of a journalist. The way to resolve differences is through dialogue, not violence or criminality.— Boris Johnson (@BorisJohnson) April 7, 2021
In a statement, the White House said it was "concerned by the violence in Northern Ireland" and joined other leaders in their "steadfast" support of "a secure and prosperous Northern Ireland in which all communities have a voice and enjoy the gains of the hard won peace peace".
Northern Ireland Secretary Brandon Lewis travelled to Belfast on Thursday to meet political parties.
Mr Lewis said he was aware of unionist concerns in recent months and he had been "engaging and listening" them.
He denied that the UK government has abandoned unionists through the new Brexit arrangements.
Labour Party leader Sir Keir Starmer said the violence was "completely unacceptable" and he called on Mr Johnson to "step up" and "show leadership".
Taoiseach (Irish Prime Minister) Micheál Martin wrote on social media: "Now is the time for the two governments and leaders on all sides to work together to defuse tensions and restore calm."
The taoiseach and prime minister spoke on Thursday afternoon.
Irish Foreign Minister Simon Coveney told BBC News NI that political leaders need to come together to make a "clear and strong statement that violence is not the answer".
"This is a time for unity in the face of violence and concern across Northern Ireland in terms of what's been happening," he said.
All of the main political parties in Northern Ireland have criticised the disorder but they are divided over its causes.
However, after an emergency meeting of the Northern Ireland Executive, a joint statement was issued from ministers, saying they were "gravely concerned" by the recent disorder.
"Attacks on police officers, public services and communities are deplorable and they must stop," they said.
"Destruction, violence and the threat of violence are completely unacceptable and unjustifiable, no matter what concerns may exist in communities.
"Those who would seek to use and abuse our children and young people to carry out these attacks have no place in our society."
The violence has been brought on by a combination of factors.
Northern Ireland politics and bigger events like Brexit are all in play here but there is no single straightforward reason.
There is a lot of fury over the attendance of Sinn Féin politicians at a funeral of a former IRA leader last summer which appeared to blatantly flout Covid-19 rules.
Sinn Féin's most senior politicians, among long lines of former IRA members, walked through the streets behind a cortege at a time when many people missed attending funerals for their own loved ones.
"Republicans do what they like and get away with it" has broadly become one of the narratives being used to justify trouble against the police.
However there were also loyalist funerals which broke rules during the pandemic.
Brexit has been creating tension in loyalist communities for months, with the growing realisation over just how differently Northern Ireland is now being treated from the rest of the UK.
Signs of anger over the new Irish Sea trade border are plastered on walls and lampposts in unionist and loyalist areas.
In these areas it feels that Boris Johnson has betrayed Northern Ireland and there is a backlash.
But that isn't all - there are much more localised issues too.
Loyalist paramilitaries have been subject to a series of raids and arrests for their criminal activities and police believe in some areas these are the figures in the background helping to encourage trouble.
Unionist political leaders - including Democratic Unionist Party (DUP) leader and First Minister Arlene Foster - have said PSNI Chief Constable Simon Byrne should resign over the force's handling of the funeral of senior republican Bobby Storey in June 2020.
Last week a decision was made not to prosecute senior Sinn Féin members who attended the funeral.
Mrs Foster tweeted on Thursday morning that she had spoken to Mr Byrne in the wake of the violence, as did Sinn Féin president Mary Lou McDonald.
The first minister had faced criticism for not speaking to the chief constable after the violence on previous days.
Mr Byrne briefed both the Stormont executive and individual parties.
Sinn Féin, the SDLP, and the Alliance Party have accused unionist politicians of ramping up rhetoric in recent days by calling for Mr Byrne to stand down.
Mark Lindsay, the chairman of the Police Federation, which represents rank and file officers, said removing the chief constable "in the middle of a crisis" would not be helpful, although he said there were "serious issues that need to be addressed".
On Thursday the Northern Ireland Assembly was recalled for politicians to consider a motion calling for an "immediate and complete end" to violence in loyalist areas.
Mrs Foster said the violent scenes were unacceptable, and Northern Ireland is "faced with a number of deep and significant political challenges in the time ahead, and collectively we must work through those challenges".
Deputy First Minister Michelle O'Neill of Sinn Féin said the violence in Belfast on Wednesday night was a "dangerous escalation" of events.
Meanwhile, Education Minister Peter Weir has confirmed the further reopening of some youth services in response to the violence.
Mr Weir said it would apply in areas of "heightened community tensions".
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