The crash happened shortly before 4pm today (January 26) on the anti-clockwise carriageway at Junction 30, close to Thurrock.
The incident has led to the closure of two lanes and lengthy delays.
Queues have formed as far back as Junction 1b of the A282 near Dartford, with severe congestion through the Dartford tunnels and up to Junction 2 at the A2 Darenth interchange.
National Highways has estimated that normal traffic conditions will not resume until between 6.30pm and 6.45pm.
Lanes three and four of the four-lane carriageway remain shut while emergency crews work at the scene.
Essex Police, Essex County Fire and Rescue Service, and the East of England Ambulance Service are all in attendance.
