Historians, politicians, writers - and singer Lily Allen - also mourned the damage to the "iconic" Charles Rennie Mackintosh building.
The Royal Institute of British Architects (RIBA) said: "Damage to a building of such immense significance and uniqueness is an international tragedy. It is irreplaceable."
President Stephen Hodder said: "The RIBA joins our colleagues in Scotland in sending out a message to the students, 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.
"It is too early to talk about what happens now, but the institute will do anything it can to help in any way."
Architect and presenter of Channel 4's The Restoration Man, George Clarke, was among thousands who went online to comment.
He tweeted: "Devastating to see the Glasgow School of Art in flames...I'll support it's full restoration in any way."
Antiques Roadshow expert Steven Moore wrote: "Seeing Glasgow School of Art on fire is heartbreaking. Its like watching a dear friend in agony & you're unable to help. I feel sick."
Architecture critic Hugh Pearman said: "This can't happen. We can't lose Mackintosh's Glasgow School of Art. It's unthinkable."
Singer Lily Allen tweeted: "Oh no! This Glasgow School Of Art fire is devastating" while broadcaster Andrew Neil wrote: "Deeply saddened that this international cultural treasure is on fire."
Scotland's Deputy First Minister Nicola Sturgeon said: "The Mackintosh Building is not just part of Glasgow's heritage - it is a national treasure."
Scottish actor Colin McCredie, who played DC Stuart Fraser in detective drama Taggart, tweeted: "I feel as if a little part of Glasgow has died today. #iconic"
Crime writer Ian Rankin wrote: "Just heard about devastating fire at Glasgow School of Art - one of Scotland's most iconic buildings. Sad sad news."