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
Ed Miliband is to outline Labour's "10-year plan" for the NHS after warning it faces "its most perilous moment in a generation".
Argentina's president has announced plans to disband the country's intelligence services following the death of a state prosecutor.
A new EastEnders trailer has been released previewing the show's anniversary special where Lucy Beale's murderer will finally be revealed.