“There is a fire at the Mackintosh Building. The fire brigade are currently on scene,” posted the Glasgow School of Art’s Twitter account around midnight, 40 minutes after firefighters were called to the scene.
More than 120 firefighters and 20 fire engines are tackling the blaze.
Around two hours after the fire was reported, a spokesperson for the fire service said the blaze is “well developed” and firefighters are “working hard” to extinguish it.
Video footage posted on Twitter shows large flames and smoke billowing into the night sky, as flashing lights from emergency vehicles can be seen in the background.
There are no reported casualties and nearby properties have been evacuated as a precaution.
In the early hours of Saturday morning, the fire spread to the nearby O2 ABC building on Sauchiehall Street.
Images and videos posted online shows the roof of the venue hall collapsing and the entire top of the building ablaze.
Jennifer Crammond, who lives around two miles away in the West End of Glasgow, is able to see the blaze from her home.
“There’s loads of white smoke and it’s extending towards another building,” she told Sky News.
She said she thought the fire “did not look contained” and that it had gotten “worse over the last 45 minutes”.
Her sister Elizabeth, an art graduate now based in England, studied at an institution with ties to the School and told Sky News: “It’s quite distressing to know that the School of Art, a place I hold quite dear to my heart, is basically gone completely.”
She described how her sister video-called her and the pair could hear an explosion come from the building.
“You could see even from her window the smoke is almost volcanic and it’s taken over Glasgow,” she said.
“It’s a worldwide piece of art history. I remember going there myself and now it’s definitely gone.
“We are devastated because we can remember what it was like and are now seeing it go down to crumbles.”
The School is located in a busy area of the city, one that is popular with students and tourists. It is surrounded by bars and nightlife spots which were packed on Friday night.
Firefighters were called at around 11.20pm on Friday.
Firefighters are on the scene, with a spokesperson commenting: “The Scottish Fire and Rescue Service is currently in attendance at a fire in the Mackintosh Building of The Glasgow School of Art.
“Operations control mobilised a number of fire engines to the scene after the call came in at 11.19pm on Friday, 15 June.
“Firefighters are currently tackling the fire.”
By 4am, deputy chief officer Iain Bushell released a statement saying: “This is an extremely challenging and complex incident, but the response and professionalism of our firefighters has been exceptional.
“The fire has taken hold several properties including the 02 ABC nightclub causing extensive damage.
“This will be a prolonged incident and crews will remain on the scene as they work to prevent further fire spread and damage.”
He urged residents in nearby buildings to remain indoors and to keep their windows closed.
Scottish First Minister Nicola Sturgeon said on Twitter: “This is clearly an extremely serious situation. My first thoughts tonight are for the safety of people – but my heart also breaks for Glasgow’s beloved @GSofA.
“Once again, we are indebted to the bravery of our firefighters and other emergency services.”
Shadow Scottish minister Paul Sweeney tweeted: “Devastated that a major fire has broken out at the Glasgow School of Art tonight.
“The Mack has been undergoing restoration since the last devastating fire in 2014. @scotfire_glasgo are doing everything they can to salvage the most architecturally important building in Glasgow.”
Scottish Secretary David Mundell said the Government “stands ready to help” the Glasgow School of Art “financially or otherwise”.
He tweeted: “Devastated to hear news of fire at Glasgow School of Art. Only fortnight since I was there to see restoration.
“My heart goes out to all those who had worked so hard on that. @UKGovScotland stands ready to help, financially or otherwise @GSofA”
The building was undergoing restoration after being damaged by a fire in May 2014. It was due to reopen in 2019.
The building was designed by artist and architect Charles Rennie Mackintosh and built between 1897 and 1909.
Buildings including the O2 ABC have suffered “extensive damage” as flames and smoke poured into the night sky from the Glasgow School of Art (GSA) building.
A restoration project, which was set to cost between £20m and £35m, had been returning the famous art school to its former glory after a devastating fire in 2014, with Brad Pitt and Peter Capaldi among those lending their support.
The “extensive” fire spread to all floors of the building including the multi-million pound restoration work, Scottish Fire and Rescue Service deputy assistant Peter Heath said.
Speaking on Saturday morning, Mr Heath said there were still pockets of fire in the building but it was not spreading. He said fresh fire crews were being deployed to relieve firefighters who have been fighting the blaze all night.
He said the operation may continue for several days.