At least four people have died in a crash on the M5, police have said.
Officers said a lorry travelling southbound crashed through the central reservation and hit at least two vehicles heading in the opposite direction.
Emergency services were called just after 14:30 BST to junctions 15 and 14 in Gloucestershire.
The motorway is closed in both directions and drivers have been asked to avoid the area.
Avon and Somerset Police said it would remain shut “for a considerable amount of time”.
The force said: “Four people are believed to have died and a number of others have been taken to hospitals in the Bristol area.”
Highways England warned of “long delays” and said there was a six-mile queue northbound between junction 18 for Avonmouth and 16 for Almondsbury.
There are also three miles of “trapped traffic” within the police cordon, it added.
A number of diversions are in place.
Cardiologist Dr Amer Hamed was in a car travelling “10 to 20 seconds behind” when the crash happened.
He was one of the first at the scene along with a passing GP and other medics to help an injured woman who was in one of the cars.
Paramedics arrived on the scene about six minutes later and the woman Dr Hamed had been helping was taken away by ambulance to hospital.
He said: “There was a lorry involved and at least two other cars. One was absolutely destroyed and another had flipped over.”
He said motorists who had been stuck on the carriageway for several hours were being “very helpful”.
“People are helping as much as they can,” he said.
“Several people offered us water and one man came out of his car to give food. We’re going to be stuck here for a few hours yet.”
BBC reporter at the scene – Chris Kelly
The most striking thing about the scene is the almost complete silence here.
Looking over the bridge, the police, fire and ambulance crews are patiently doing their work at quite a disturbing scene.
The lorry that has crashed has hit what M5 commuters will know as a familiar landmark, a windsock next to the road. But alongside that are several cars crushed from the impact.
On the other side of the bridge, motorists patiently wait with some eager to help. I overheard one asking if any prayer support was needed. While others talk to police officers about the sadness of what has happened.
South Western Ambulance Service said it was called just after 14.30 BST.
A spokeswoman said: “We have 13 resources at the scene including the critical care team and the hazardous area response team.”
Avon Fire & Rescue Service sent six vehicles.
Highways England said the “early expectation” is that the M5 “will remain closed in both directions late into this evening”.
It added drivers on the M4 planning to join the M5 northbound were unable to do so.