The incident, which was first reported at about 3.32pm, involved a road ambulance, an air ambulance and North Yorkshire Fire & Rescue Crews to the A172 near Ingleby Cross.
Motorists were facing long delays due to the incident.
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Multiple people were cut from their cars by fire crews using hydraulic equipment and motorists are still advised to avoid the road if possible.
A spokesman for North Yorkshire Fire & Rescue Service said: "Crews remain at the scene of this three vehicle road traffic collision. A number of people have been released by crews using hydraulic cutting gear.
"Fire crews from Northallerton, Stokesley and two from Cleveland Fire attended a three vehicle road traffic collision. Crews extricated trapped occupants who were then treated by Ambulance crews at the scene.
"Please avoid the area where possible."