At least seven people have been killed and 51 injured in a devastating motorway pile-up said to be among the worst UK collisions in memory.
Police said the death toll from the crash on the M5 in Somerset last night was expected to rise as the recovery operation continued.
Some 34 vehicles were involved in the accident, which happened in wet and foggy conditions and was described by emergency workers as "the worst road traffic collision anyone can remember".
Assistant Chief Constable Anthony Bangham of Somerset and Avon Police said: "Seven people are confirmed to have died as a result of this tragic incident and we do believe there could be more deaths.
"Our thoughts are with those who may have lost loved ones as a result of last night's collision.
"A large-scale multi-agency operation continues at the scene to remove vehicles, check that there are no other casualties and repair the carriageway. This is a hugely complex operation because of the scale of vehicles and people involved.
"All vehicles will need to be removed from the scene for forensic examination and this of course takes time."
Of the casualties, 25 were taken to Yeovil District Hospital and 17 of the more seriously injured to Musgrove Park Hospital in Taunton, he said. Nine others were taken to a temporary holding unit set up by the ambulance service.
Chief Constable Colin Port, who visited both the site and casualty bureau overnight, also gave his condolences to those who had lost loved ones.
"I would like to say a huge thank you to officers from all the emergency services who have had to deal with this very difficult and traumatic incident," he added.
"Everyone has pulled together, including members of the community, local businesses, all our partners and all those outside of Avon and Somerset to provide help and support to those involved."
Support units were still at the scene this afternoon to carry out searches of nearby fields and verges around the collision site.
The motorway remained closed in both directions between junctions 24 and 25 following the incident, which took place at about 8.25pm last night on the northbound carriageway.
Mr Bangham said: "Once the carriageway is clear a large operation will take place to repair the road surface and clear fuel spillages. It is anticipated the motorway will not open until tomorrow at the earliest.
"However we will endeavour to keep the public informed as soon as we can."
Meanwhile dramatic television footage of the aftermath showed vehicles melted into the road in a scene resembling a war zone.
The crash, which triggered a "massive fireball", took place close to the Bridgwater Guy Fawkes Carnival and it was suggested that smoke from the event could have worsened the fog on the road.
Police could not say whether the fireworks display might have been a factor, but Mr Bangham said it was "certainly something we'll be looking at closely".
He said: "There's a number of factors that came into play. It was dark - it was about 8.25pm - it was particularly poor weather last night, we had fog banks on the motorway and we also had wet surface issues.
"We had in particular ground water in the Bridgwater area which we were dealing with.
"There were also other factors coming into play: there were events going on in the evening and of course we need to have a very close look at what was going on in the area that may have caused some sort of distraction."
For a motorway incident this was, he said, "just about as big as it gets".
He added: "It was a highly complex, very traumatic incident as well for our services and officers."
Some members of the public had also shown "real bravery" in arriving quickly on the scene and trying to help others, he said, thanking those who had been in the area and taken actions themselves.
"The intensity of the fire - it was a fireball on the carriageway - made it incredibly difficult for people to approach," he said. "People did their very best."
The intensity of the fireball was "quite unusual", he added.
Emergency services were continuing to search for casualties well into the day, both in vehicles and in the nearby carriageway.
A mixture of heavy goods vehicles and light vehicles were involved, and a number were in close contact, burnt and unrecognisable.
Mr Bangham described it as a "real mash of many, many vehicles" and said it was believed that some people were still trapped inside.
He added: "The goods vehicles were well alight and have been very significantly burnt."
Police were contacting families of those caught up in the incident and people who were concerned that their loved ones had not returned home.
"The most important thing is we support the families and look after their needs in this most difficult time", Mr Bangham said.
He appealed for witnesses to send in any mobile phone footage of the incident that they might have taken, as emergency services worked together to carry out what was a "very, very significant operation."
A "comprehensive and thorough" investigation into what caused the tragedy will now be carried out, Mr Bangham said.
Describing the incident earlier, he said: "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."
Firefighters who scrambled around 15 appliances to the scene battled to rescue motorists by cutting people from cars and lorries using hydraulic equipment, while shocked witnesses described the inferno and ensuing carnage.
Simon Bruford, 38, from Williton in Somerset, who was driving south, told the BBC: "I could see the flames from quite a way back.
"I spent 18 years in the Somerset fire service and have seen a lot nasty things, but that was horrific."
Paul O'Connor, who was travelling to Plymouth when the incident happened, told Sky News: "I thought it was something to do with bonfire night and then realised it was something quite bad.
"I have never seen anything like that. I could see people lying on the side of the road. It was quite disturbing really.
"I saw two people lying down and there were quite a lot of people around them. The emergency services were doing what they could. I don't know if they were OK."
Local resident Bev Davis heard the accident 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 were explosions of what I think must have been tyres - it was as though the fireworks were starting again but we knew they had finished.
"There must have been 200 metres-worth of fire - plumes of smoke were going up and everything was red."
Weather forecasters said conditions had been misty and any bonfires burning nearby could have made things worse.
Gareth Harvey, a forecaster at MeteoGroup, the weather division of the Press Association, said: "The particles bonfires release encourage fog droplets to form.
"By 9pm there were (weather) stations in the county reporting visibility down to 100 metres. The roads would also have been wet due to an earlier deluge."
Transport Secretary Justine Greening expressed her "deepest sympathies" with the families affected.
"I would like to thank the rescue workers and emergency staff who worked throughout the night to help the injured," she said.
"The police have the full support of the Highways Agency in establishing exactly what happened."
Mayor of Taunton Deane Steve Brooks and borough council leader John Williams also said they wished to extend their sympathy and support to all those involved.
Mr Brooks said: "My heart goes out to everyone affected by this terrible crash. They and their families will, I know, be in the hearts and prayers of the people of Taunton Deane.
"I would also like to extend my deepest and most sincere thanks to the emergency services who showed the ultimate professionalism. They have the most difficult jobs at times like these."
Mr Williams added: "Whatever Taunton Deane Borough Council can do to help those affected we will do. The incident and effect on those involved is unimaginable."
Edmund King, president of the AA, said the scale of the crash was similar to one on the M4 near Hungerford in Berkshire in 1991.
In foggy conditions 10 died and 25 were injured in a 51-vehicle smash, he said.
Long tailbacks resulted from the part-closure of the M5 during the day, with some drivers stuck in stationary vehicles for up to 30 minutes at a time.
The worst problems were experienced on the A38, running parallel to the closed section of the road, according to travel information provider Trafficlink.
Delays were also reported on the A538, which was being used as another diversionary route through Taunton.
Long queues developed on the northbound and southbound carriageways approaching the closed section of the M5, a Trafficlink spokeswoman said.
Anyone who did not need to travel in the area today was advised not to.
As a result of the pressure on the road network - and as a mark of respect - the North Petherton Carnival due to be held nearby tonight was cancelled.