Loyalist paramilitaries supposed to be on ceasefire organised major rioting in Belfast and opened fire on police, a senior officer has revealed.
Around 500 people were involved in violence at a sectarian interface in the east of the city last night.
Police said shots were fired from the republican Short Strand area, while loyalists also opened fire, but masked Ulster Volunteer Force (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 bullet marks on police vehicles were blamed on the UVF and are being treated as the attempted murder of 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."
The violence is being described as some of the most serious seen in Northern Ireland for years.
The UVF is one of the biggest loyalist paramilitary groups and despite having observed a ceasefire and having decommissioned its weapons, the group was blamed for a murder last year.
A paramilitary watchdog group 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 on the area.
Last night's violence was condemned at the Northern Ireland Assembly, and while local representatives gave conflicting accounts of what happened, 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."
Petrol bombs, bricks and other missiles were hurled during the rioting, which comes ahead of the most tense period of the loyalist marching season.
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.