Essex Police said officers were called shortly before 3.50am on Monday to a report that two people had climbed on to the Queen Elizabeth II Bridge at the Dartford Crossing and were “currently at height”.
Just Stop Oil said in a statement that two supporters of the group had scaled the structure “to demand that the government halts all new oil and gas licences and consents”.
An Essex Police spokesman said: “The bridge has been closed while we resolve the incident which we will do as quickly and as safely for all involved.
“It does mean that we have had to close the bridge to traffic, but a diversion is going to be put in place through the tunnel.
“This is likely to cause delays throughout this morning and this incident may take some time to resolve due to the complexities of safely getting people down from height.”
By 6am on Monday, the force said there were “delays of around 60 minutes”.
National Highways East wrote on Twitter at 7.11am that there were “120 minute delays with 5 miles of congestion on the approach”.
Just Stop Oil said that “two climbers ascended the two 84m masts on the north side of the Queen Elizabeth II Bridge forcing the police to stop traffic from entering the bridge”, adding “it is expected that the bridge will remain shut for at least 24 hours”.
Morgan Trowland, 39, a bridge design engineer from London, said in a statement released by Just Stop Oil: “As a professional civil engineer, each year as I renew my registration, I commit to acting within our code of ethics, which requires me to safeguard human life and welfare and the environment.
“Our government has enacted suicidal laws to accelerate oil production: killing human life and destroying our environment.
“I can’t challenge this madness in my desk job, designing bridges, so I’m taking direct action, occupying the QE2 bridge until the government stops all new oil.”
Another protester, identified only as Marcus, 33, a teacher from London, said: “Only direct action will now help to reach the social tipping point we so urgently need.”
The A282 Dartford Crossing is currently the only way to cross the Thames east of London by road.
The 2.8km-long (1.7 mile) QEII Bridge southbound and two 1.4km-long (0.8 mile) tunnels northbound link Essex and Kent.
The A282 also connects directly at both ends with the M25 London Orbital Motorway, one of the busiest motorways in Europe.