Emergency services were called to the A36 near Bath at 11.30am following reports a 52-seater coach had failed to stop and had left the road, plunging 30ft from a viaduct.
When the emergency services arrived, seven passengers had walked free from the wreckage.
The driver and a passenger were trapped inside and had to be freed.
Those who were trapped are understood to be seriously injured. All nine people involved were taken to hospital.
Members of the band Baroness are understood to have been travelling on the coach, as part of their European tour.
More top news
Downton Abbey actress Joanne Froggatt is to star in a new ITV drama exploring the devastating cost of deceit.
It's been 12 years in the making but the spacecraft Rosetta's mission has finally come to an end. Here is what you need to know about it.
The boss of DreamWorks has hinted that a fifth Shrek movie is in the pipeline saying there is "more to come" from the franchise.