Several vehicles were involved, including a lorry.
"There were many people with blood on their bodies and then the vehicle that had been forced down into the ditch with the lorry was surrounded by many men who were trying their hardest to get out the people inside... the emergency services were incredible but honestly it was the worst crash I've ever seen".
Huge queues built up behind the crash scene after the motorway was closed in both directions.
The force said the collision happened between junction 16 near Almondsbury and junction 14 near Falfield.
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He was one of the first at the scene alongside a passing GP and different doctors to help a harmed lady who was in one of the autos.
South Western Ambulance Service Trust said a dozen ambulances and a critical care team were working at the scene.
The lorry that has crashed has hit what M5 commuters will know as a familiar landmark, a windsock next to the road. The southbound carriageway is also closed but has been cleared of traffic and diversions are in place.
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Traffic tailed back for up to six miles as emergency services came to the aid of the injured.
A statement read: "Road users further afield intending to travel through this area should consider alternative routes and allow plenty of extra time for their journey".
Interstates England said the "early desire" is that the M5 "will stay shut in the two bearings late into tonight".
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