It is feared more than 10 people have died in a 27-vehicle crash on the M5 near Taunton, Somerset.
More than 40 people were injured in the crash, near junction 25 northbound at about 20:30 GMT on Friday, which police said led to a "massive fireball".
The cause of the crash is not yet clear but there had been heavy rain in the area earlier and there was patchy fog.
Avon and Somerset Police have released a helpline number for people concerned about relatives: 0800 092 0410.
The Highways Agency said the stretch of motorway between junctions 24 and 25 was not expected to reopen until 21:00 on Saturday evening.
On Friday night, drivers reported delays of up to five-and-a-half hours and motorists have been urged to avoid the area altogether.
Musgrove Park Hospital, in Taunton, said it was treating 16 casualties who had suffered "a range of trauma injuries", including broken bones and chest, abdominal and lung injuries.
Colin Close, the medical director at the hospital, said none of the casualties were in a critical condition and all were stable.
"I think it's fair to say we've never seen anything like this in this hospital before," he said.
A specialist surgeon from the Royal Devon and Exeter hospital was drafted in to help deal with the injured at the Taunton hospital.
Yeovil District Hospital said it dealt with 26 "walking wounded" and had discharged 24 of them by Saturday morning.
A man and a boy remain in the hospital but their conditions are not thought to be life-threatening.
"It was a very busy night for us and our staff did really well," said a spokesman.
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.
"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 six articulated lorries involved in the accident, as well as lighter vehicles.
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."
Coach driver Michael Wagner said he had "never seen anything like this".
He said: "There were lots of lorries on fire. There was a car underneath one of the lorries.
"It looked like two lorries had jack-knifed at the front. In all, there must have been eight or nine lorries and 15 cars behind them. There was smoke and flames everywhere."
He said there was debris "everywhere, even on the southbound side".
Jaysen Sharpe, who had been at nearby Taunton Rugby Club which had been hosting a fireworks display at about the time of the crash, said he had seen "a number of explosions, petrol tanks that I believed were going up".
Motorist Steve Burge said he had seen two lorries on fire and a car which had been almost burnt out.
"It was something that you wouldn't want to see," he said. "It was an image that will stick with me forever."
Driver James Starkey, who was caught up in the tailbacks about 500 yards from the crash, said a "huge, billowing tower of smoke" began rising from the scene at about 21:00 GMT.
He said police had told him traffic would be turned round as soon as possible "but with the amount of vehicles to turn and get moving it could be a long time".
"The atmosphere among people waiting on the motorway is one of shock," he added.
The Highways Agency said the motorway closure had affected people leaving the Bridgwater Guy Fawkes Carnival.
North Petherton Carnival, which was due to be held on Saturday night, has been cancelled.
Andrea Day, from Trafficlink, said one of the busiest routes on Saturday would be the A38 through Taunton and warned of disruption throughout the weekend. Traffic is reported to be stop-start on the road.
A spokesman for frozen food retailer Iceland confirmed one of the firm's lorries had been involved in the crash.
"We are working closely with all of the authorities to ascertain the circumstances of the incident and to help in whatever way we can," he said.
Taunton Deane MP Jeremy Browne said it had been a "horrendous accident".
He said: "There has not been a crash on this scale for many years and the implications of it will be life-changing for many people."