Political leaders appealed for calm after 500 people were involved in the disturbances in east Belfast last night that saw hand-to-hand fighting, plus the use of petrol bombs and blast bombs.
Police said there were gunshots from the republican Short Strand area, while loyalists also opened fire, but masked UVF members were blamed for starting the violence by attacking homes in the Catholic enclave.
Two men on the loyalist side of the divide suffered gunshot wounds to the leg, officers confirmed.
But two bullet marks on a police vehicle were blamed on the UVF and are being treated as an attempt to murder officers.
Chief Superintendent Alan McCrum said: "We believe at this point that members of the east Belfast UVF were involved.
"It would be a line of investigation to establish whether that was a co-ordinated and organised 'organisational' position (by the UVF central leadership).
"But at this point we are satisfied that at the very least members of east Belfast UVF were involved in organising the disorder."
Sinn Fein said scores of masked men in camouflage clothing and wearing surgical gloves were at the centre of co-ordinated attacks on the republican Short Strand area that ignited a five-hour riot.
Homes were damaged and there were a number of injuries, with witnesses claiming it was lucky no one was killed.
Northern Ireland First Minister Peter Robinson and deputy First Minister Martin McGuinness condemned the riot, as well as a separate bomb attack aimed at police in west Belfast.
Mr Robinson said: "At this time when many are working hard to build a better and brighter future for all in Northern Ireland, it is disappointing and deeply concerning to see this level of violence return to our streets."
He added: "We have given clear commitments to continue to deliver progress for all within the community including in those areas most at need. This type of behaviour damages the local economy and unfairly mars the reputation of the community."
Mr McGuinness said: "A small minority of individuals are clearly determined to destabilise our communities. They will not be allowed to drag us back to the past.
"I call on all those involved to take a step back and to remain calm. I support the efforts of community leaders on all sides who have been working on the ground to restore calm in east Belfast."
He added: "We need to ensure that tensions are not raised over the coming weeks, and I will do everything I can to assist community leaders in their efforts to bring calm to the streets."
The sudden upsurge in violence is being described as the worst the city has seen in years and loyalist community workers blamed simmering tensions at the notorious sectarian interface.
But other observers blamed rivalries inside the UVF, fuelled by anger at restrictions placed on contentious parades, plus the efforts of police to probe crimes from the Troubles as part of an ongoing review of cases by the Historical Enquiries Team.
The UVF is one of the biggest loyalist groups and despite having observed a ceasefire and having decommissioned its weapons, it was blamed for a murder last year.
A paramilitary watchdog found that the UVF's leadership sanctioned what was branded the "public execution" of loyalist Bobby Moffett who was shot dead in front of shoppers on Belfast's Shankill Road.
But the Independent Monitoring Commission (IMC) stopped short of recommending government sanction of the UVF.
The recent appearance of UVF murals in east Belfast depicting masked and armed men was seen as a bid by the group to stamp its mark.
The location of the riot is an inner city area, not far from the centre of Belfast, and has been a long-standing flashpoint.
The Short Strand is a small Catholic community in the predominantly Protestant east of the city.
Local representatives who witnessed the night's disturbances gave conflicting accounts of what happened, but the police said the episode was initiated by the UVF.
Mr McCrum said: "It started when a group of young men after nine o'clock last night made their way into the area of the Short Strand and did unquestionably attack homes in that community.
"That precipitated a response from the community in the Short Strand and then we were left with two communities who then for the next four hours were seeking to involve themselves in conflict across what was, and continues to be, a very challenging interface."
At the height of the disturbances, republicans fired six shots, while loyalists fired five shots. Loyalists were blamed for opening fire on a police Land Rover, leaving strike marks on the vehicle.
The police said they would step up security in the area in the nights to come, but it is being noted that the incident raises the temperature ahead of the most volatile period of the loyalist marching season.
Asked about the shots fired by republicans, Mr McCrum said there was no indication of Provisional IRA involvement.
"There is nothing to suggest at this point that those shots were fired by Provisional members," he said.
He paid tribute to the police, who he said were in the front line trying to protect communities against crowds that were "hell-bent on disorder". Police fired a number of baton rounds.
In a further violent incident, police came under bomb attack from dissident republicans in west Belfast.
As officers were responding to a report of a suspicious vehicle in the Andersonstown Road area in the early hours of this morning, a bomb was hurled at their car.
Northern Ireland Policing Board chairman Brian Rea condemned the bombing, as well as the east Belfast riot.
He said the officers in west Belfast had a lucky escape: "It is extremely fortunate that officers were not injured in this attack but this device had the potential to kill which was the obvious intent."
Justice Minister David Ford said the violence was a disgrace and showed Northern Ireland in a negative light, following two days of positive publicity around the US Open win by Co Down golfer Rory McIlroy.
"Northern Ireland has occupied the headlines for our sporting excellence but that positive image, bringing with it the prospects of continuing to improve our reputation internationally, is today replaced with one of rioting," he said.
