Traffic was halted at the Dartford Crossing due to a “police incident”.
At its peak, there were three miles of congestion heading into Kent on the QEII Bridge, just before Junction 1a for Dartford.
Traffic cameras show heavy traffic on the motorway near Thurrock.
On X, formerly Twitter, a spokesperson for National Highways said: “The A282 QEII Bridge is currently blocked southbound between the M25 Junction 31 to Junction 1a due to a police-led incident.
“Emergency services are making their way to the scene.”
It is currently unknown if anyone has been injured.
One motorist said police vehicles and ambulances have gone past them in the queue.
The route has since reopened, but there are currently delays of more than an hour.
Emergency services have been contacted for comment.
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