Police and community representatives are appealing for calm after trouble in west and north Belfast on Monday night in which 22 officers were injured.
An investigation is under way into the possibility that shots were fired in west Belfast during the trouble.
An ambulance and a fire engine were also attacked.
Community worker Robert McClenaghan, from the Falls Residents Association, said the violence was organised by nationalist youths.
More than 40 petrol bombs were thrown at police, who said crowds of about 150-200 people were involved in the violence at Broadway and about 200 in both the Oldpark and New Lodge areas of north Belfast.
Crowds of nationalists threw petrol bombs and masonry at police who fired 51 plastic bullets in the Broadway and Oldpark areas.
Mr McClenaghan said the violence was "nothing but thuggery".
"There was no provocation. There was nothing from the Protestant, loyalist, unionist community. Broadway interface was quiet," he said.
"What you had was 100 or 150 of these young people all tooled up and masks around their faces.
"They were armed with petrol bombs, they were armed with sticks and stones. They had wheelie bins with extra ammunition that they were pulling behind them."
A bus was hijacked and driven at police during the disturbances which occurred before the annual loyalist Twelfth of July demonstrations
A number of arrests were made.
Police said an ambulance crew was attacked while attending a hoax call in Brighton Street off the Falls Road.
A fire engine had its windscreen smashed by youths throwing bricks and bottles whilst attending a fire at the side of the Glen Road in west Belfast.
While four officers received hospital treatment, none of the injuries are believed to be life-threatening.
On the Falls Road, in the west of the city. A driver was dragged from his bus and the passengers ordered off.
It was then driven at police lines on Donegall Road, but crashed a short distance away. A van was also set alight on the Donegall Road.
Assistant Chief Constable Dave Jones praised his officers for their "skill and professionalism" in dealing with the violence.
"Unfortunately 22 officers sustained injuries while delivering this protective service," he said.
"We would appeal for everyone to do everything they can to help ensure all areas are calm and peaceful over the next 48 hours. Violence does not need to be inevitable."
Sinn Fein MLA Jennifer McCann was at Broadway for several hours during the trouble. She believes that those taking part in the rioting were not from the area.
"Mostly what I saw was people who came from other parts of Belfast, who would be known as anti-social elements, who had taken a lot of drink and were attacking the police," she said.
"I am appealing to young people who may be caught up in this behaviour to stay away from Broadway, Ardoyne or anywhere else there may be a chance of trouble."