Assistant Chief Constable Anthony Bangham, of Avon and Somerset Police, said "several people" lost their lives.
"The officers and the other emergency services faced a very, very difficult scene when they arrived," he told the BBC.
"Many vehicles were on fire and the collision itself involved what we believe to be about 27 vehicles, both lorries and cars.
"So they were faced with virtually all of them on fire. Many of them have burned literally to the ground." Around 35 people were believed to have been injured, he said.
Local resident Bev Davis saw 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 told the BBC.
"There must have been 200m worth of fire - plumes of smoke were going up and everything was red."
Motorist Paul O'Connor described the scene as "horrific". "I have never seen anything like that - I could see people lying on the side of the road," he told Sky News television. "It was quite disturbing really."
Paul Slaven, of Devon and Somerset Fire and Rescue Service, said the fire was likely caused by the number and type of vehicles and he had no indication they were carrying any toxic or chemical material.
"At least two of the vehicles on fire were articulated lorries and there would be a lot of fuel on them," he said.
The exact death toll was unconfirmed last night.
-AAP