Vehicles have been hijacked and torched amid violence linked to Eleventh Night bonfires.
A suspected pipe bomb also detonated close to a police operation to clear a contentious fire site in Belfast while other security alerts prompted the closure of main roads, with one incident preventing newly-arrived passengers exiting Belfast City Airport.
Masked men also used burning cars to block roads close to the Ulster Hospital in Dundonald on the outskirts of Belfast, while a bus with passengers on board was hijacked before being set alight in Newtownards.
In Derry, two "crude but viable" devices were left on the city walls.
Around 16 petrol bombs were thrown at police close to the walls, while debris was set alight in the Lecky flyover.
Last night's violence unfolded amid a police warning that loyalist paramilitaries were planning to "orchestrate and participate in serious disorder" in east Belfast.
Assistant Chief Constable Alan Todd said a total of 13 vehicles were burnt in Belfast and Newtownards.
“No part of this is acceptable and today we are looking at the evidential picture to pursue lines of enquiry," he said.
“I would like to pay tribute to the job that my officers, along with other emergency services, did last night.
“The disorder we have seen has been caused by a small minority of people who have no regard for their communities. No one wants to see disruption and disorder and the people behind these incidents need to ask themselves just what they are achieving."
The violence came after loyalists expressed anger at moves to limit the size of bonfires at two controversial sites in the east of the city.
By 1am this morning, the Northern Ireland Fire and Rescue Service (NIFRS) had received a total of 327 emergency calls, mobilised to 164 operational incidents - 57 of which were bonfire-related.
Fire officers said the figures represented a 23% increase on incidents attended last year, with the regional control centre at times handling an emergency 999 call every 48 seconds.
Fire appliances came under attack on three occasions, at Carn Hill in Derry, Moygashel near Dungannon and Ballycarry/Whitehead. No firefighters were injured.
There had been community tensions in east Belfast over the size of the Bloomfield Walkway and Cluan Place pyres, with many residents fearful of potential damage to their properties.
Loyalists claimed they had taken steps to minimise risk to buildings this year and characterised attempts to restrict bonfires as attacks on their culture.
Video from the scene:
On Tuesday evening, a High Court judge had ordered that the Bloomfield Walkway bonfire be reduced in height.
Police moved into the area in the early hours of yesterday morning, but loyalist youths set the structure alight before any material could be seized.
The Cluan Place bonfire was toppled and cleared, with police escorting masked contractors hired to do the job. During that operation a loud bang was heard in the area and the remnants of a pipe bomb were later recovered close by.
Police said the threat from the UVF in east Belfast was directed at officers.
PSNI Assistant Chief Constable Alan Todd said last night: "I would strongly urge people to desist from engaging in any violent or criminal behaviour.
"I would also appeal to those who have influence in this community to discourage people from taking part in any illegal activity.
"The safety of the public and my officers is of paramount importance and accordingly I will be reviewing my resourcing plans to ensure there is an appropriate and proportionate policing operation in place to deal with any disorder should it occur."
In Newtownards, Co Down, a bus carrying passengers was hijacked by masked men, who were believed to have been carrying firearms.
None of the passengers were injured in the incident which took place around 7.30pm on Wednesday.
The bus was hijacked in Blenheim Drive and later set on fire.
Police also attended the scene of nearby Messines Road which was blocked by burning tyres.
On the Upper Newtownards Road in Dundonald a number of masked males pushed a car into the road and set it on fire close to Robbs Road at around 9.45pm.
A further report was received that two cars were set on fire close to the nearby Carrowreagh Road. Both roads are close to the Ulster Hospital, which has a busy Accident and Emergency Department.
A PSNI spokesman said: "We would like to the thank the members of the public who are currently assisting us as we try to identify these individuals who appear determined to obstruct the road network, particularly so close to the Ulster Hospital's Accident and Emergency Department."
There were a number of other security alerts in the east of the city through the night.
One forced the closure of the Sydenham bypass dual carriageway, meaning passengers were unable to exit the adjacent Belfast City Airport for a period.
A large section of the Newtownards Road - one of the main thoroughfares in the east of Belfast - was closed due to a suspect car.
The road re-opened this morning.
