The west wing, and possibly more, of the A-listed masterpiece, designed by Charles Rennie Mackintosh and one of the country's most admired and best loved buildings, was gutted in the conflagration.
At first, before the full fury of the fire became evident, all onlookers could see of the blaze, which began at around 12.30pm, was thick plumes of smoke, billowing from windows on the west side of its famous main entrance.
Firefighters and two fire engines from the Scottish Fire and Rescue Service stations in Cowcaddens, Yorkhill and Maryhill arrived within minutes, and, wearing breathing apparatus, led a number of students and staff to safety.
The cause of the fire has yet to be officially determined, but many witnesses believe it began in the five-storey building's basement when a hot or faulty film projector set fire to expanding foam being used in a student art show.
Students recounted how the fire caused by the projector quickly spread, despite the efforts of a member of staff to put it out with a fire extinguisher.
The extent of the damage, though evidently considerable, remained unclear last night.
Aerial photographs suggest that the interior and roof of the west wing of the building has been lost, including Mackintosh's famous library, student studios, archives, furniture, busts and statues, wooden panels, art displays, the director's office, and the Hen Run, a glazed corridor which ran the length of the building at roof level.
The Mackintosh Library housed large books from the GSA Special Collections, publications on and by the school's staff and students, and graduating students' degree show catalogues.
Rare and archival items, including art periodicals dating back to the early 19th century and publications about Charles Rennie Mackintosh, were also housed there.
By early afternoon, a crowd gathered at the scene and watched with alarm as flames suddenly burst from first floor studio windows.
By 1.30pm the blaze was widespread.
Smoke was also seen rising from the south side of the building and as firefighters began to pour water into the building with high pressure jets, flames leapt from other windows and the building's roof.
Muriel Gray, the television presenter and writer and chair of the GSA board, arrived at the scene, clearly distressed.
Onlookers, students and staff, gasped as gouts of red flame burst up from the roof and licked the stone exterior of the landmark building.
Ms Gray was not the only onlooker distressed as the flames spread, both across the building and the roof, and the crowd were moved progressively further away from the burning building by police, who cordoned off Renfrew Street.
Students had spent the day bringing in their materials for the forthcoming Degree shows, for a deadline of 5pm.
Robert Mills, 21, a second year Fine Art Photography student, was helping to install a show in the room adjacent to where he believed the fire began.
He said: "I would not say a projector was faulty but it got hot and set fire to some foam. I was nearby and we all just got up and moved so fast.
"Everyone just got out. The fire alarms went and we all moved.
"There were so many flammable things there, and it just went up."
Student Tara Marshall-Tierney, 20, who also gives tours of the Mackintosh Building, witnessed the fire break out.
She said: "We were in the basement when the fire started. We heard a pop and then the fire started going up.
"I'm just speculating, but it seems likely to have come from an overheated projector.
"One student passed the fire extinguisher to our tutor, Paul Cosgrove, who is the head of sculpture and environmental art. He tried to put it out but he couldn't.
"Everyone tried to do something but we couldn't stop it, so we got out. It was smoking quite fast and then it suddenly spread to all four walls.
"The alarm went off and people were shouting 'Fire! Fire!'"
Last night, Scotland's Chief Fire Officer Alasdair Hay said it was too early to pinpoint the cause of the fire, and that specialist teams were working with Police Scotland to establish what started it.
He said in total 17 fire units from Glasgow were at the scene at the height of the blaze as well as specialist units from across the country.
He added: "Part of our operation is to try and salvage what we can and we worked with colleagues from the school who identified objects of significance that they would like us, if possible, to save.
"They described the objects and their location and we briefed officers, and firefighters, where they could, did the salvage if it was safe to do so."
Kelvin Holdsworth, Provost of St Mary's Cathedral in Glasgow, was close by when he saw the fire and smoke.
"You could see the fire burning behind the Mackintosh grills on the windows," he said.
"The smoke was coming initially from one side of the building.
"It is a terrible day for the city."
Stuart Robertson, director of the Glasgow-based Charles Rennie Mackintosh Society, said: "It's absolutely heartbreaking.
"It's like losing a limb. The school is totally unique.
"It's an art school but it's also a work of art in itself. Lots of people have been walking round in tears."
Structural engineers from Glasgow City Council were last night working with fire crews to try and stabilise the fire-damaged building.
The fire service said it was working with staff to try and save key works of art from the building.
Gordon Matheson, Leader of Glasgow City Council, described the fire as "heartbreaking."
He said: "The council will provide all practical help and assistance that it can. We will work hand in hand with The School of Art in an effort to save the building and the works of art contained in it."
Roddy Buchanan, winner of the inaugural Becks Futures Prize 2000 and one member of a generation of award-winning artists who trained at the school, said: "It is the most important architectural space in the world for me.
"Former students who lived in it know it to be special, and no-one was ever precious about the building.
"I suppose if and when it is now restored there is a danger that there might be a lobby not to have students in there and that would be wrong."
The fire came only the day after the building's new neighbour, the Reid Building, was named as Building of the Year by the Architects Journal.
Iain Connelly, the president the Royal Incorporation of Architects in Scotland, said: "The value of this building goes well beyond Glasgow or even Scotland.
"Scotland has seen the loss of an international treasure which reflects the genius of one of our greatest ever architects and the whole of the architectural profession in Scotland will, I am sure, join with me in sending out a message to the students and staff of the School and all those who have been associated with this building over the decades, a message of sorrow and commiseration at this terrible, terrible news."
Both the Culture Secretary, Fiona Hyslop, and Janet Archer, chief executive of Creative Scotland, also expressed their dismay at the fire.
Ms Archer said: "We have lost a vital archive, art collection and library, and a vast amount of student work which will now never see the light of day. It's a terrible event for Glasgow, Scotland and for the world."
Deputy First Minister Nicola Sturgeon said: "The Mackintosh Building is not just part of Glasgow's heritage - it is a national treasure."
The whole Art School site will remain closed until Monday.