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
Philip Thomas doesn't like to dwell on the disaster that buried him alive - but he can still recall in detail how an unpaid debt saved him.
The deal would set up a free trade zone between the EU and Canada - and could be an important test for the UK's Brexit negotiations.
A mother has won her battle against the Home Office to allow her Nigerian sister into the country - so she can undergo a marrow transplant.