Up to 30 people have been injured, seven of them seriously, in a crash on the M5 in Somerset involving 26 vehicles.
Police said they believed "people have lost their lives" in the crash, between junctions 23 and 26.
Two lorries were among four vehicles which are said to have caught fire in the accident shortly before 20:30 GMT.
One witness described the scene as "horrific". The 18-mile stretch of the motorway is due to shut for 24 hours.
Motorists are being asked to avoid the area.
A spokesman for Avon and Somerset Police said: "There are reports of between 25 and 30 injuries, 10 are believed to be life changing and it is believed that people have lost their lives."
A spokesman for Devon and Somerset Fire and Rescue said: "There are reports of up to 30 casualties, including several people seriously injured."
He said he could not recall having a road traffic collision on this scale for some time in this area.
Around 50 firefighters are at the scene along with 15 fire engines from Taunton, Glastonbury, Bridgwater, Wiveliscombe, Wellington, Burnham on Sea and Yeovil.
Simon Bruford, 38, from Willerton, in Somerset, said he was driving southbound when he passed the scene with his wife and son.
He described the scene as one of "total carnage".
"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."
Tim Shires, 29, from Bristol, said he got caught in the southbound traffic on his way to Plymouth for work.
He said a police officer was warning drivers not to expect to be moving for quite a few hours.
"We've been told a couple of helicopters could land on the carriageway," he said.
"I've heard helicopters up above but nothing has landed but it's foggy and visibility is poor."
Avon and Somerset Police said the M5 was "likely to be closed for 24 hours" and appealed for drivers to avoid the area because of "huge tailbacks".http://www.bbc.co.uk/news/uk-england-somerset-15603124