A full-scale emergency exercise has been held to test the response to a major incident inside the northbound tunnels at the Dartford Crossing.
Only northbound M25 traffic was affected by the live exercise
The two tunnels which run under the River Thames between Dartford in Kent and Thurrock in Essex were closed from 0100 BST to 0600 BST on Sunday.
A simulated incident tested the emergency services, Highways Agency and local authorities in Kent and Essex.
Diversions were put in place during the five-hour safety exercise.
Northbound traffic had to leave the M25 at junction two, follow the A2 through the Blackwall Tunnel, and then take the A102 and A13 back to junction 30 of the motorway.
Vehicles over 4m (13.1ft) in height and those carrying hazardous goods had to travel clockwise around the M25 to reach their destination.
Southbound traffic using the QEII Bridge over the Thames was unaffected.
John Aspinall, from the Highways Agency, said: "It is vital to ensure that in the case of a real emergency all the parties involved operate in an efficient manner to safeguard the public.
"Holding Exercise Orpheus means the authorities' response to a real emergency would be swift and practiced."