It is believed a rear wheel of the First-operated bus fell into a hole in the road caused by subsidence from a burst water main.
The incident happened early this afternoon.
Eyewitness Billy Murdoch, 60, from Glasgow, said: “They have had engineers out taking photos of this since it happened.
“It looks like subsidence and the bus was tilting at quite an angle.”
The bus was stopped in a bus lane outside George House when it began to sink. No passengers were on board at the time.
A cordon was set up around the bus and police attended the scene.