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
Old Labour firebrand Derek Hatton is now locked in a new fight - to rejoin his old party.
After a career which began with carrying Britain's first nuclear bombs, the last remaining Vulcan Bomber is being retired.
Tonight will see further showers across northern Britain, whilst Wales sees rain arriving overnight pushing eastwards to many central areas