The world famous Glasgow School of Art has suffered devasting damage after a fire ripped through the listed Charles Rennie Mackintosh building.

Firefighters are still struggling to control the blaze, which appeared to have started in the basement of the building shortly after midday. Eye witnesses feared that the whole west wing of the old building has been lost, including the irreplaceable Mackintosh library and the Hen Run, a famous corridor running along the roof which linked the west and east wing.

The school was completed in 1909 and is considered by many to be Mackintosh's masterwork.

Students and staff crowded into the streets surrounding the art school in Renfrew Street, Glasgow city centre, at lunchtime as the fire took hold. Many wept and comforted each other as they watched the fire extend upwards through the floors, with flames ultimately billowing from the roof of the Category A listed building.

Throughout the afternoon, the level of the destruction started to become apparent. Glasgow fire service and art school officials said they were as yet unable to assess the scale of the damage.

Seekee Zhang, a second-year painting and print-making student, was in the canteen in the newly opened Reid building opposite when the fire broke out. "We could see smoke coming out, and we didn't know what was going on, then the windows started to crack."

"The building was evacuated, and we started to see the flames coming out behind us. It was shocking. There are a lot of people crying."

The library inside the Glasgow School of Art's Charles Rennie Mackintosh building. Photograph: Barry Lewis/In Pictures/Corbis

Zhang said that the majority of the old building had been cleared out to accommodate the upcoming final-year degree show. The deadline for submissions was 5pm on Friday, and many students were in the building working on their presentations.

They included Anna Sundt, who fought back tears as she explained: "I was four hours away from finishing a four-year degree." She said that all her research, documentation and work was in the burning building. "Each degree student was creating their own individual studio in the old building. Of course the most important thing is that everybody is safe, but it's the opportunity that everyone working in there has now lost. It was our one opportunity to show friends, family, the art world our work, and it's all gone."

Among the students gathered at the corner of Rose Street and Renfrew Street, there was speculation that the fire had been started by a spark from a projector, possibly igniting some foam-based material. One student suggested the fire took hold so quickly because the building had been very recently painted.

Alasdair Hay from the Glasgow fire service said that crews had responded within four minutes of receiving the original 999 call, and that the fire service had "brought resources from across Scotland" to deal with the fire. Seventeen units attended in total. He refused to comment on suggestions from some students that a sprinkler system had yet to be installed in the old building. He also said it was impossible for crews to assess the scale of the damage as they were still working to fully extinguish the fire.

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Muriel Gray, chair of the board of governors of the art school and a former pupil herself, arrived at the scene as soon as she heard the news. "I don't know what to say. This building is so important to the world. It's one of the most important buildings in my life."

Later, speaking at a hastily arranged press corner on the corner of Rose Street and Renfrew street, she thanked the fire service, the staff and students of the art school and the people of Glasgow for their good wishes throughout the afternoon. "I am so proud of the staff and students and how everyone has pulled together. We are thankful to all the Glaswegians who turned up to comfort students and to friends from across the world for their messages of support."

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," while sympathy and offers of help in restoring the building poured in from around the world.

Douglas Moreland, an artist and tutor at the art school, lingered in the corner of Rose Street into the evening. "I just feel so helpless. I wanted to run up there myself with buckets of water." The destruction of the Mackintosh building would send out shock waves worldwide, he said. "It is a world-class loss. It's so much more than just a building. It's something to do with the Glaswegian identity and the energy associated with it. I still keep thinking that I'm going to wake up from the nightmare in a second."