Drivers on the M25 faced miles of gridlock after a serious crash near the Dartford Crossing brought traffic to a grinding halt.
Motorists reported being stuck on the motorway in barely-moving traffic for more than three hours on Friday morning.
The QE2 bridge between Essex and Kent was completely shut clockwise and heavy traffic stretched back over eight miles on either side.
One person was seriously injured and rushed to hospital following the crash between a lorry and a car at just after 4.40am.
A man was arrested on suspicion of dangerous driving and is in custody, Kent Police said.
Highways England said the bridge reopened at 1.40pm, adding that although queues were easing motorists should still allow extra journey time into the afternoon.
Earlier, traffic which would have gone across the bridge was diverted through the eastern bore of the Dartford Tunnel.
Motorists took to social media to discuss the delays.
“Three hours in, travelled two miles, thank you M25,” one social media user wrote online, adding: “Total gridlock.”
“Almost three hours in car and still not crossed river,” another post on Twitter read. “Traffic moving very slowly.”
Other drivers reported sitting in the car with the engine off because the roads were so congested.
“The A282 Dartford Crossing QE II bridge is closed due to a serious road traffic collision involving a HGV and a car,” Highways England said.
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“Traffic is being diverted around it by using a contra flow in the tunnels. Southbound traffic is currently being diverted through the east tunnel and northbound traffic will continue to use the west tunnel at the Dartford Crossing.
“Due to heavy congestion in the area road users may wish to consider alternative routes and are also advised to allow extra time for their journey.”