One of the demonstrators is a “bridge design engineer from London”.
Morgan Trowland, 39, said: “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.”
Trowland was accompanied by a 33-year-old teacher from London known only as Marcus.
Trowland has been arrested several times following Extinction Rebellion protests.
Essex Police said the bridge would remain closed throughout Monday and into Tuesday morning’s rush hour.
Chief Superintendent Simon Anslow said: “We understand how frustrating this situation is for people wanting to use the bridge and go about their business. Let me assure you we are doing all we can to resolve this situation quickly and safely.
“It is a complex situation due to the height the people on the bridge are at.
“Our colleagues at National Highways, who have responsibility for the bridge, have made an assessment that it is not currently safe to re-open the road.
“We will take action against anyone who puts life in danger or thinks they can selfishly stop others getting to and from work or just going about their daily lives.”