Two hundred people have been injured as more than 100 vehicles were involved in a series of crashes in thick fog on the A249 Sheppey crossing in Kent.
Six of those hurt in the collisions, at about 07:15 BST, are thought to have serious injuries.
Early reports suggested up to 40 vehicles were involved, but police later said more than 100 had crashed.
Firefighters said they had freed five people from their vehicles on the southbound carriageway.
About 200 people have minor injuries.
One witness said visibility was very poor but drivers were approaching the crossing with no lights.
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 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.
Ch Insp Andy Reeves said the crash was over a "protracted area" with undamaged vehicles between others which had collided.
He said there were a number of injuries but no-one had died.
Several people, who were still in their vehicles, may have suffered serious injuries, he added.
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