Police have arrested an 18-year-old man on suspicion of arson following a fire in Belfast city centre on Monday morning.
The "suspicious fire" broke out in the Church Street area of Belfast at around 5.40am, police said.
Fire crews have been tackling the blaze all morning at the Old Cathedral Building in the heart of Belfast's Cathedral Quarter.
By 9am, two aerial appliances could be seen aiming hoses at the smoking roof of the building which sits opposite St Anne's Cathedral.
More than 50 personnel have been involved in fighting the fire.
Detective Sergeant Dougherty said: "The fire is currently being treated as suspected arson at this time and we have arrested an 18-year-old man on suspicion of arson endangering life with intent. He remains in police custody at this time.
"A number of diversions are still in place for motorists and we would advise the public to avoid the area where possible."
SDLP councillor Carl Whyte said: “I visited the site of the fire on Monday morning to offer my support to those affected and to establish if any help was needed in the interim.
Understandably those who use this space are very distressed and temporary accommodation is available for businesses at the Cathedral Quarter Business Improvement District.
“This is one of the most historic parts of our city centre and it’s devastating to see this building damaged as a result of this blaze.
There has been much discussion about the need to revive this part of our city in recent months and this is the last thing the area needs.
“I’d like to pay tribute to the work of the fire service who have been working tirelessly at the scene from the early hours of Monday morning. The council will work with all those who were based at the building to find emergency accommodation as soon as possible and to address the fallout from this fire.”
Translink said all Metro and Ulsterbus services would be avoiding the area until further notice.
While no flames remained visible this morning, there was a thick smell of smoke in the air as hundreds of students watched in disbelief as they filed into the new Ulster University campus.
Tina Calder is the owner of Excalibur Press which is based in the area.
She told BBC Good Morning Ulster her brother who works as a security guard alerted her to the fire by 6am.
"I panicked, I came straight down to see the whole place was starting to get engulfed in flames over the course of an hour," she said.
"Over the course of the next hour I just watched as the whole roof just was engulfed in flames. I watched as one of my colleagues in the building, their office went up in flames.
"It seems to now be under control in the fact that there's no visible flames but we know there's going to be a lot of water and smoke damage.
"All my staff are now working from home today. I spoke to one of the firefighters...I was trying to ask him where it started.
"Obviously he has to be careful, but they feel it may have started somewhere near the roof or the back of the building."
A Northern Ireland Fire and Rescue Service (NIFRS) said: “Eight fire appliances from Belfast, a Command Support Unit from Lisburn, two aerial appliances, 52 firefighters and five officers are in attendance.”
Firefighters working to contain a blaze which started early this morning at the Old Cathedral Building on Donegall Street in Belfast. Multiple appliances at the scene while the surrounding streets have been closed off. @irish_news pic.twitter.com/oxyok4PEp9
— Allan Preston (@AllanPreston) October 3, 2022
Devastated for our neighbours in the Cathedral Building on Donegal Street this morning. An iconic building full of great businesses on a neglected street is not what the area needs or deserves. Hope everyone is ok pic.twitter.com/TEgYzcHskD
— The Deer's Head (@TheDeersHead1) October 3, 2022
#Belfast city centre - fire in Cathedral Quarter continues - York St now closed to traffic at Jct Great Patrick St / Frederick St - motorists & pedestrians advised to avoid the area - @NIFRSOFFICIAL attending (08:42)
— Trafficwatch NI (@TrafficwatchNI) October 3, 2022