Eight of those hurt in the collisions, which took place at about 07:15 BST, have serious injuries.
Early reports said 200 people had been hurt, but police later revised the figure.
Firefighters said they had freed five people from their vehicles on the southbound carriageway.
One witness said visibility had been very poor at the time of the crash but drivers were approaching the crossing with no lights.
Others at the scene described a mass of tangled cars, lorries, and a car transporter. Some reports said the crash went on for 10 minutes as cars continuously collided with each other.
Driver Martin Stammers said the scene was “horrendous” and described seeing cars under lorries and people lying on the floor.
He said visibility was about 10 to 20 yards when he approached the bridge and saw five cars smashed into each other with one across the outside lane.
He said he managed to squeeze through a gap between that car and the central reservation, he said.
“For 10 minutes afterwards, all we could hear was screeching, cars thudding into each other, lorries crashing,” he added.
He said he and his son ran to warn other drivers and warn cars to slow down.
“Later, a woman came up to us sobbing saying, ‘thank you, thank you, you saved my life’,” he said.
‘Smashed cars everywhere’
Jaime Emmett, a 19-year-old student who was driving through the fog, said she managed to stop in time but a van collided with her car, and she then hit a car in front.
She said the fog was so thick she could only see a few cars ahead but added: “All I could hear was the cars smashing in front of each other and I could not know how far ahead the accident was.”
Later she said she could see “smashed cars everywhere”, a lorry that had crashed into the central reservation, and ambulance crews helping injured people with one team carrying a man in a stretcher.
Ch Insp Andy Reeves said the crash was over a “protracted area” with undamaged vehicles between others which had collided.
The injured people were taken to six different hospitals.
When asked if the fog had caused the crash, Mr Reeves said it was “too early” to give a cause but added the “weather will be a factor”.
“It was… very hazardous. It was described to me as a very thick fog and it was certainly low visibility at the time.”
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