Part of the building was destroyed by fire four years ago
A huge fire has broken out at Glasgow School of Art, just four years after it was severely damaged in a blaze.
More than 120 firefighters and 20 fire engines have been tackling the blaze.
Around two hours after the fire was reported, a spokesperson for the fire service said the blaze was "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 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.
There is a fire at the Mackintosh Building. The fire brigade are currently on scene. We will report back with news updates as soon as possible.— Glasgow School Art (@GSofA) June 15, 2018
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 show the roof of the venue hall collapsing and the entire top of the building ablaze.
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.
Glasgow Art School on fire again. Gutting! ☹️ pic.twitter.com/MVMPz4HMym— Rocco Giudice (@81Rocco) June 15, 2018
Firefighters were called at around 11.20pm on Friday.
Firefighters are at 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."
Glasgow School of Art is on fire beware those on suchihall street pic.twitter.com/lBNduZJeru— KC (@CrossleyKieran) June 15, 2018
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 of 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."
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.
Given the severity of the blaze, restoration work is likely to have been destroyed and the firefighting operation may continue for several days, Mr Heath said.
The fire officer said the blaze was a "devastating loss for Glasgow," with firefighters dealing with a "sense of loss" along with the city's people.