"It was rightly judged to be the best building of the past 175 years in a nationwide poll run by the RIBA.
"Damage to a building of such immense significance and uniqueness is an international tragedy."
The Scottish Fire and Rescue Service said the indications were that 90 per cent of the structure remained "viable". It added up to 70 per cent of the building's contents had been saved.
Glasgow-born architect and designer Mackintosh (1868-1928) was a leading exponent of art nouveau.
He won a competition to design the building in 1897 and it took about 10 years to complete but is now a landmark in the city, with special government-protected status.
The school's alumni include recent Turner Prize for art winners Simon Starling (2005), Richard Wright (2009) and Martin Boyce (2011).
Others include Doctor Who actor Peter Capaldi, Harry Potter and James Bond movie actor Robbie Coltrane, and members of Scottish rock bands Travis and Franz Ferdinand.
- AAP