Scores of firefighters were tackling a "well-developed" blaze that had consumed the Glasgow School of Art's Mackintosh Building on Friday night, the second time in four year a serious blaze has hit the historic structure.
Images posted on social media showed flames and smoke billowing from the roof of the renowned art school, one of Scotland's most famous structures.
Fire ravaged the building in May 2014 and a multi-million pound restoration project has been returning the famous art school to its former glory.
The Scottish Fire and Rescue Service said the blaze at the Mackintosh Building in the Glasgow School of Art was "well developed" and firefighters were "working hard" to extinguish it.
A spokeswoman said they were called at 11.19pm and a number of fire engines were mobilised.
"The Scottish Fire and Rescue Service is currently in attendance at a fire in the Mackintosh Building of The Glasgow School of Art," she said, adding they were not aware of any casualties.
Graduation ceremonies had taken place earlier on Friday, while the May 2014 blaze came in the run-up to students' degree show.
Nicola Sturgeon, Scotland's First Minister, said: "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."
Fire 'much more extensive' than 2014
Shadow Scottish minister Paul Sweeney, Labour and Co-operative MP for Glasgow North East, said he was "devastated" a fire had consumed the building again.
He added: "The 2014 fire destroyed around 20% of the western wing of the building - a later extension completed in 1909, containing the spectacular library - with a further 30% smoke damaged. This fire appears to be much more extensive and affects the 1899 eastern wing of the building."
He later said "the entire interior space is now fully alight. The best we can probably hope for is structural facade retention and a complete rebuild of the interior".
There must be "a comprehensive national effort to ensure every possible option to salvage and restore what is one of the finest edifices in the history of world architecture is pursued in the wake of this latest setback"," he said.
"I'll be doing everything I can to promote that in Parliament," he said.
Susan Aitken, the city council leader, called the fire "devastating".
Witness Aidan Dick said the fire and smoke grew in intensity "in the space of a few minutes".
He tweeted: "Glasgow School of Art is on fire again. Heartbreaking. My thoughts go to all students and staff, and I hope no one was caught in the blaze."
Police said a number of roads around the building were closed, including part of Sauchiehall Street from Charing Cross to Rose Street, while Renfrew Street from Garnet Street to Cambridge Street has also been shut. The public were urged to stay away from the area.
Shock at 'terrible' blaze
Connor Neil said people were being evacuated from their homes and there was a "big orange light" which could be seen from streets away.
The 22-year-old chef from Glasgow said: "We've got just to the top of Renfrew Street and police closed off the road.
"You can just see a big orange light coming from the school and people are being evacuated from their homes."
Alex Kapranos, singer of Franz Ferdinand, was among those on social media reacting to the "terrible" blaze.
Fran Healy, singer for the band Travis, said it was "so so sad" to see the building on fire again.
The institution is one of Europe's leading art schools, while the listed Mackintosh-designed building is a tourist attraction in its own right.
Construction work started on the building in 1897 after Mackintosh won a design competition while working for Glasgow architects Honeyman & Keppie. It took around 10 years to complete due to funding issues and several alterations were made by Mackintosh during the process.
More than a century later, it still operates as a working building serving hundreds of students.
Peter Swanton, a former Glasgow School of Art student who also witnessed the 2014 fire, said it "hurts" to see the Mackintosh in flames again.
"The Glasgow School of Art was a home away from home for 4 years and still such an important part of my life. To see this from my roof hurts," the designer wrote on Twitter.
"I was standing in front of the Mac 4 years ago as it burned. I had just finished my first year of art school. I never thought I would see the same scenes being repeated."
Local resident John McInnes, 22, said: "I can't believe that this has happened again to such an iconic building. It's unbelievable. Questions need to be asked, they spent so much on renovation, but this seems so much worse than the last one."
Restoration project
The Glasgow School of Art had gone to extreme lengths to restore the Mackintosh building after the 2014 fire.
The school described itself as "custodians of an iconic piece of world architectural heritage" with a responsibility to restore it for students and staff.
After the blaze significantly damaged the west wing, including the loss of the celebrated Mackintosh Library, the first stage of the restoration was to fit a temporary roof over the site.
A full-scale model of part of the library was later built to test if the original materials and techniques from Mackintosh's 19th century design could be used in its reconstruction.
The specifications for the model bay were developed with "meticulous research" into every aspect of the original design and construction of the library. To rebuild the wooden library, the restoration team sourced wood from the US which was the same age as that used in Mackintosh's 19th century design.
The first phase saw outer work with damaged stonework repaired and the roof replaced before the delicate process of restoring the west wing and upgrading the east wing interiors began last year.
Hollywood star Brad Pitt and Dr Who actor Peter Capaldi were made trustees of The Mackintosh Appeal to support the rebuild.
Pitt is a fan of building designer Charles Rennie Mackintosh and Capaldi is a former art school student.
Work was due to be completed in February 2019.