It is a blessing no-one seems to have been hurt.

With flames reaching high into the air from the upper windows of Glasgow School of Art's Charles Rennie Mackintosh building yesterday, it was clear that the fire had spread quickly, apparently from the basement, and would have posed a serious risk to life had staff and students not responded so promptly when the alarm to evacuate sounded.

Now that the immediate drama is over, however, the tragic devastation of Rennie Mackintosh's much-loved and internationally venerated masterpiece - and with it the precious degree work of numerous students - is hitting home. This is a different sort of shock and sadness, but one that is reverberating across Scotland and beyond, touching the legions who revere or simply delight in the work of the great Scottish architect and designer. Their distress was visible yesterday outside the art school, where tears flowed freely, and on social media. The art school is both a unique work of art and a piece of history that, by definition, cannot be replaced.

It is regarded by international consensus as the acme of 20th-century architecture. If a series of works by Van Gogh had gone up in flames, it would be no more painful than this.

The school's destruction by fire, a poignantly Victorian fate for a building designed at the close of the Victorian era, prompted an anguished statement from the Royal Institute of British Architects, which ran a poll in 2009 naming the building as the most important of the last 175 years. It summed up the global significance of the Rennie Mackintosh building by calling the fire an international tragedy.

And so it is. It is too early as yet to know the extent of the destruction but it would appear that much of the building's interior, which Rennie Mackintosh designed in every detail, has been severely damaged, though firefighters made efforts to salvage what they could. The whole west side of the building, including the precious Mackintosh library, has been gutted. How much of the shell can be saved remains to be seen. The drive now will be to maintain as much as possible of what stands and probably restore the rest as faithfully as humanly possible. A fundraising campaign to support the work would surely garner massive support, given the huge affection in which the building is held.

An investigation will establish how the fire started, amid reports that a projector overheated. This was a building full of wood, plaster, paper and paint and perhaps one of the most important questions for any inquiry will be to look at what sort of systems existed in the building for dealing with fire; there is uncertainty about the role, if any, of a sprinkler system.

All that, however, is for the weeks to come. Today, dozens of students will wake to the jolting memory of yesterday's events and the harrowing realisation that their work has gone. For them and the staff who have lost their creations, this is a very personal tragedy. Their work cannot be replaced but it is fervently to be hoped that the dear building they had the privilege and pleasure of calling their place of study will be saved.