Michael Wagner captured footage of the crash scene on his mobile phone minutes after the collision
"Several" people were killed and up to 35 injured in a 27-vehicle crash on the M5 near Taunton, Somerset, police say.
Officers said the crash, which happened close to junction 25 northbound at about 20:30 GMT on Friday, led to "one massive fireball" at the scene.
Emergency crews said the cause of the crash was not clear, but there had been reports of heavy rain and fog.
Avon and Somerset Police have released a helpline number for people concerned about relatives: 0800 092 0410.
Continue reading the main story
Travel News - Check your journey here
The latest traffic and travel information for Somerset
The Highways Agency said the stretch of motorway was not expected to reopen until 21:00 on Saturday evening.
The M5 is currently shut northbound between junctions 25 and 24, and southbound between junctions 23 and 25. Some drivers reported delays of up to five-and-a-half hours and motorists have been urged to avoid the area.
A spokesman for Musgrove Park Hospital, in Taunton, said it was treating 16 casualties who had suffered "a range of trauma injuries".
Assistant Chief Constable Anthony Bangham, of Avon and Somerset Police, said "several people" had died but he could not yet be specific about numbers.
He said: "The incident was very, very challenging and on arrival crews were faced with literally one massive fireball.
The crash is expected to close part of the motorway until Saturday evening
"Most vehicles were well alight and most continued to burn for a considerable time. This made it very difficult to search the vehicles. Some of them have been burned to the ground."
Emergency crews said they believed the weather may have been a contributory factor in the crash, with witnesses describing having seen thick fog at the time.
Paul Slaven, of Devon and Somerset Fire Service, said crews were "damping down" at the scene of the crash, which he said was "the worst road traffic collision anyone can remember".
He said: "There is a large amount of debris covering the road and a couple of vehicles are still smouldering after being on fire so the police have said the road will be closed for at least 24 hours and possibly more for the carriageway to be cleared."
The fire service said there had been 20 "light vehicles" involved in the incident, along with six articulated lorries.
Motorist Steven Swift, who passed the accident scene while heading south about 30 minutes after the crash, said he had seen three lorries on fire and some injured people lying in the central reservation.
He added: "It was a horrific scene. The heat was intense."
Bev Davis, who had been in a field near the motorway looking after her horses, said: "There was this horrific sound of a horn. The horn was just permanent.
"You knew something had gone on and then the flames were just so high so quickly and the noise was horrific. And then there were explosions, I think probably of tyres going."
She added: "From where we were standing we could only see what was directly in front of us because the flames and smoke were smothering everything. There must have been 200m-worth of fire - plumes of smoke were going up and everything was red."