Firefighters are struggling to bring the fire under control at the Glasgow School of Art Mackintosh Building. They were called out at around 12.45pm today.
Eyewitnesses saw a large plume of smoke gathering over the city centre as the fire took hold and said flames were streaking in to the sky. Windows have also been blown out. People have been treated for smoke inhalation.
Murial Gray, a former student of the Art School, has been spotted outside the building, visibly upset.
A Scottish Fire and Rescue spokeswoman confirmed crews were at the scene, but added no further information was yet available.
Fire Scotland chief Alasdair Hay and assistant Fire Scotland officer David Goodhew have arrived at the scene, with one source saying it is unusual for the head of the organisation to attend an ongoing incident.
The fire started in the basement around about lunchtime. All the students were evacuated at about 12.20pm.
Alex Misick, 25, was helping his girlfriend put the finishing touches to her degree show project across the road in the school's new Reid building when they saw plumes of smoke.
The GSA graduate, now vice president of the Students' Association, said: "I think everyone is in shock. It's soul-destroying.
"It was literally a few hours to go before everyone finished up and handed in. That's everyone's work over three or four years. Everyone's in tears."
Austin Yuill, who works as a chef at the art school, told the BBC: ''I've been moved two streets away from the Mackintosh building but before we left the place was completely ablaze all down the west side of the building.
''I'm told it started in the basement and it's worked its way all the way up through the five floors.
''As far as I know it started from a spark which has gone on to foam, expanding foam.''
Asked how busy the art school was, he said: ''It would be very busy because we're working up to the end-of-year assessments, so all the students were installing their work today all over the Mackintosh building. There are a lot of very upset students here.''
He added: ''Quite apart from it being voted the best building of the last 175 years, it is a major tourist draw and has an incredible reputation as an art school. This is really terrible.''
Scotland's First Minister Alex Salmond said on Twitter: "Thoughts with staff & students at @GSofA - awful to see destruction of this iconic building and students work."
Scottish Culture Secretary Fiona Hyslop said on Twitter: "Serious concern about fire at Glasgow School of Art."
Shadow Scottish secretary Margaret Curran used the social networking site to say she had "just heard about terrible fire at Glasgow School of Art", adding: "Hope people are safe."
Scottish Tory leader Ruth Davidson, who is an MSP for the Glasgow region, said the blaze was "dreadful news", describing the art school building as "one of Glasgow's finest".
Senior firefighters said the operation at the scene was likely to be "protracted".
While there are no reports of anybody having been injured, the fire service confirmed that it did lead several people to safety.
In its latest update, the fire service said crews remain in attendance and have been working to bring the fire under control with high pressure water jets.
A spokesman said: "Appliances from across Glasgow and west Central Scotland were mobilised to the incident, which began shortly before 12.30pm.
"Firefighters reached the scene within four minutes of the service receiving the first of several 999 calls.
"Search-and-rescue teams entered the building wearing breathing apparatus and led a number of people to safety.
"Both internal and external firefighting operations have been conducted throughout the incident and three aerial rescue pumps are currently being used to douse the flames from height. There are no reports of injuries at this point."
Chief Officer Alasdair Hay, who is at the scene, said: "This is likely to be a protracted incident and crews have been working extremely hard to tackle what is clearly a very significant fire.
"The priority throughout this operation has been to protect life but salvage operations are also under way."
Fran Healy, who studied at GSoA before becoming lead singer of guitar band Travis, expressed his sadness on Twitter.
The Why Does It Always Rain On Me? singer said: "Man the mac library is all wood!!!! And loads of paper!! Man!!
"Just super super sad this place which has been with Glasgow and with all the students is burning down..."
But he also paid tribute to the fire fighters who had been sent to contain the blaze.
Architect and television presenter George Clarke, who had studied Mackintosh, also paid tribute to the structure.
He said: "Devastating to see the Glasgow School of Art in flames...I'll support its full restoration in any way.
"Thank God no one was hurt today...but my heart does go out to all the students who have lost their amazing work!"