Police say it is "truly miraculous" that no one was killed in a 100-vehicle pile-up on the New Sheppey Crossing Bridge which left hundreds of people injured.
Witnesses described the accident at around 7.15am as "mayhem".
And Chief Inspector Andy Reeves, from Kent Police, said: "From my perspective, it's truly miraculous. It's very fortunate."
A Kent Police spokeswoman said visibility was a problem at the time, and witnesses say there was "very, very thick" early morning fog hovering over the bridge.
There are reports of some motorists driving "like idiots" in the conditions before the crash that closed the A249.
Police re-opened the bridge shortly after 5pm.
Witnesses told Sky News that some motorists were driving without headlights at the time of the accident.
At least eight people have suffered serious injuries, and another 60 people suffered minor injuries.
Lives are thought to have been saved by a quick-thinking lorry driver who used his truck to block the entrance to the bridge and stop more cars piling into the crash.
A driver involved in the incident, Chris Buckingham, told Sky News: "He was going the other way and ... he's gone down to the end of the carriageway, gone across the roundabout and actually blocked off the road so no more cars could actually enter the dual carriageway before the emergency services got there.
"Whoever that guy is I'd like to shake his hand because he's probably saved lives."
Up to 30 medical response vehicles were on the scene and motorists have been warned to avoid the area.
The bridge was clogged with buckled cars, lorries and even a car transporter as people waited at the side of the road to receive help from the emergency services.
A witness to the pile-up, Martin Stammers, told Sky News: "I was very, very, lucky. I was the last car out of it, if you like. As I came to the top of the hill, there were about five cars already smashed up, one was across my carriageway.
"I had to hit my brakes hard as well, I just had enough space to get through.
"From then on, all you could hear was the screeching of car tyres and the thudding, which was endless.
"It must have been going on for five to 10 minutes. You could hear the screeching, you could hear the lorries thudding into cars, you could hear glass breaking, there was nothing we could do.
"Even after the police turned up, you still heard further down the bridge – a quarter of a mile, half a mile away - cars still going into the back of each other. It was horrendous."
Student Jaime Emmett, 19, was driving through the fog when she became involved in the pile-up.
"There was a man at the side of the road saying to stop. I stopped in time but a van smashed in to me and I smashed in to the car in front," she said.
"I was lucky I was not injured. It was all quite surreal when it happened."
Another witness, David Ingram, described the scene of the crash as "mayhem".
"People were going quite fast, too fast. We were about 50 to 60 cars back. Luckily people coming on to the island were flashing their lights and waving their arms like mad (to warn us).
"It's carnage, it really is. The fog was very, very thick today. You could not see a vehicle in front of you as you came on to the bridge.
"We managed to stop .. the people in front of us weren't that lucky. It was mayhem."
Valentine Elad, a 46-year-old teacher whose car was struck from behind in the crash, told of the eerie aftermath of the pile-ups.
He said: "There were cars upside-down on other cars. There was a black four-wheel-drive Mitsubishi upside-down on a small white car, and an Audi upside-down on the bonnet of another car. It was horrible."
A statement from the Kent Fire and Rescue Service said: "There are no fatalities but ambulance crews are dealing with a large number of walking wounded casualties.
"Firefighters have used hydraulic cutting equipment to release five people from their vehicles."
AA president Edmund King said the pile-up may have been caused by "stupid driving".
"It's really bad to travel too close to the car in front in good conditions and if you do it in foggy conditions it's an absolute recipe for disaster.
"In dense fog you cannot see the brake lights ahead. By law, you don't have to have fog lights on, although it's recommended.
"Unfortunately many people don't know how to turn their fog lights on. You may only need them once a year but it's vital they get used."
A statement from Kent Police said: "Emergency services are currently at the scene dealing with the incident.
"Officers are urging motorists to avoid the area but if a journey to the Island is essential, the old Kingsferry Bridge remains open but expect long delays."