The accident, which happened in thick fog with some witnesses saying that visibility was down to 20 yards in places, was attended by nine ambulances and six fire engines.
According to reports the accident continued for 10 minutes as cars and lorries crashed into each other including a family with a nine-month-old child who had had a narrow escape.
It is believed to be the biggest accident in fog in living memory with around 200 people seen by medical staff at the scene on the London-bound carriageway travelling off the Isle of Sheppey.
Lives were probably saved because an unidentified quick-thinking lorry driver used his truck to block the entrance to the bridge and stop more cars piling into the crash.
Chief Inspector Andy Reeves, from Kent Police, said: "It is truly miraculous. Looking at the extent of the incident, the number of vehicles and people involved, the damage to some of those vehicles, I think we're all very fortunate there were no fatalities."
Mr Reeves said it was too early to say what had caused the accident but the heavy fog, which often shrouds the bridge, and driver behaviour are possible factors.
There were reports of some motorists driving "like idiots" in the conditions before the crash, which completely closed the A249 that goes over the seven-year-old bridge that opened in July 2006.
Police would also look at the lack of lighting on the structure - a design issue that has been highlighted by the area's Conservative MP Gordon Henderson.
Speaking at the scene, Mr Reeves said: "In conditions of thick fog, a significant number of vehicles have been involved in a number of collisions.
"It has been ascertained that 100 vehicles have been seriously damaged in this incident, or series of incidents. In addition, a number of other vehicles have also been caught up in it.
"Visibility was very bad, I understand visibility was down to 25 metres in thick fog, and clearly visibility will be a key factor in our considerations, but at this stage we haven't yet determined what caused the start of the crash."
Mr Reeves added: "I've been doing this job over 20 years, I've never seen anything of this size or scale before."
Witness Martin Stammers, 45, from Minster, told Kent Online: "It's horrific. I've never seen anything like it in my life.
"All you could hear was cars crashing. We got out of our car and it was eerily quiet, with visibility down to just 20 yards."
Those injured were taken to local hospitals, including Medway Maritime Hospital in Gillingham which said it had cancelled planned surgery for the day to concentrate on those caught up in the crash.
It has treated five people for serious injuries.
Kent Police said there were collisions at the top of the bridge and at the foot of the approach to it.
Work to clear the road has now started with some motorists waiting hours to get back to their cars.
- The Independent
