“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 two hours after the blaze 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.
More than 60 firefighters are tackling the blaze.
No casualties have been reported so far.
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.
There are reports that people have been evacuated from their homes in the area.
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.”
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.”
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.