Two fuel tankers and more than 20 other vehicles crashed in thick fog in County Durham, leaving a scene of "absolute devastation", firefighters said today.
Police dealing with the series of three different incidents on the A19, which left at least one person seriously injured, said the weather was a "contributory factor" in what happened.
Officers said drivers had been going too fast in the conditions.
Durham Police said the incident began with a collision near Peterlee on the southbound carriageway shortly before 10.15pm yesterday.
This initial incident is thought to have involved a petrol tanker, a diesel tanker, two lorries and a car.
Officers said one person travelling in the car, which was trapped between the two tankers, was seriously injured.
They said it was a difficult operation for paramedics and fire crews to get to the casualty as the car was so close to the tankers, one of which was leaking.
As police were dealing with the incident on the southbound carriageway a second series of collisions took place on the other side of the A19 involving 18 vehicles.
A third crash then took place on the Wellfield slip road involving three vehicles, one of which was a Vauxhall Astra police car involved in directing traffic.
A police spokesman said there were not thought to be any casualties from these two subsequent incidents.
He said: "Thick fog on the east coast is thought to have been a contributory factor in what happened and police are urging drivers to avoid the area."
The spokesman said the officers on the scene "also say too many motorists are failing to take heed of the conditions and are simply going too fast."
Firefighters were quickly on the scene as crews at nearby Peterlee Fire Station heard the initial impacts.
Dave Turnbull, the fire officer in charge, told BBC Radio Newcastle: "My crews heard a series of loud bangs.
"When they responded they found absolute devastation on the fly-over adjacent to the fire station - 20 to 30 cars, a large number of HGVs, a petrol tanker and a large number of people trapped."
The A19 remains closed in both directions and is likely to remain closed for some time, police said.
Officers from Durham are working with colleagues from Northumbria and Cleveland Police to investigate the series of crashes.
Durham Police has set up a casualty bureau for concerned relatives.
The number for the bureau is 0191 386 0098.