At least seven people were killed and 51 injured, police said Saturday, in one of the biggest British motorway crashes in decades, with an inferno burning vehicles to cinders.

The 34-vehicle pile-up at 8:25 pm (2025 GMT) Friday on the M5 near Taunton, southwest England, sparked explosions and a fireball, with lines of cars and trucks engulfed in flames.

Police said they expected the death toll to rise, with further casualties feared to be still trapped in their vehicles.

Blazing trucks fell on their sides and people desperately tried to prize open the doors of cars to help trapped motorists escape in chaotic scenes after the crash, which happened in foggy and wet conditions.

Witnesses said they heard explosions as vehicles went up in flames and saw debris and casualties strewn across the road.

A large section of the primary route through England's southwest remains closed in both directions.

"This is a horrific scene," Assistant Chief Constable Anthony Bangham of the Avon and Somerset Constabulary told reporters on a bridge over the motorway.

"We've had a collision involving 34 vehicles, many of which are burnt -- and burnt literally to the ground.

"I am also able to confirm that we've got 51 casualties. Many of those have gone to hospital.

"Sadly I can now confirm that we believe we've had at least seven people die as a result of this incident.

"We fear that that number will rise as the day continues and we carry out an extensive search of vehicles on the carriageway at the scene."

While casualties have been taken to two nearby hospitals and some treated at the scene, Bangham said others may still be trapped.

Edmund King, president of the Automobile Association, said it was Britain's worst traffic accident in two decades, the last comparable incident being a 51-car crash on the M4 motorway in March 1991 in which 10 people died.

Hospitals said the injuries ranged from simple limb fractures to more complex chest and abdominal trauma.

Bangham said detectives would be examining the possible factors involved, including the darkness, fog and groundwater, plus a fireworks display close to the three-lane road.

"We need to have a very close look at what was going on in the area that may have caused some sort of distraction," he said.

Local resident Bev Davis described seeing a wall of flames at the crash site from her home close to the motorway.

"All we could hear was the sound of a horn and then the flames got so high so quickly and the noise was horrific," she said.

"There must have been 200 metres (660 feet) worth of fire -- plumes of smoke were going up and everything was red."

Motorist Paul O'Connor told Sky News television: "It was quite horrific and I have never seen anything like that -- I could see people lying on the side of the road."

The motorway will not reopen before daytime Sunday, with police needing to complete their forensic work before the carriageway can be repaired.