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
A British couple with a combined age of 194 are to become the world's oldest newlyweds when they marry in the summer.
Weekend weather starts wet for some
Manchester United manager Louis van Gaal says Ryan Giggs will almost certainly succeed him at Old Trafford.