Eyewitness Eucharia Amarachi Innocent-Anunobi told BBC Scotland she had been “stuck” on a bus near the scene since about 06:50, but has now been diverted through Bankfoot.
She said: “At first there were no police but lots of police and fire engines have now arrived.
“Police officers approached our coach and said the road would be closed all day because of the level of damage the fire caused.”
She said the bus had been diverted through Bankfoot, where there was now very heavy traffic.
A spokesperson for Police Scotland confirmed the road was closed in both directions and diversions are in place.