Insulate Britain say 14 protesters have walked onto the carriageway of the M25 in several locations.
Liam Norton from Insulate Britain said: "In a couple of days COP 26 will start in Glasgow and the eyes of the world will be on this country.
"Britain should be leading the world with radical plans to decarbonise our society.
"What we have instead is a budget that is yet another act of treason by this government upon its own people.
"It is a plan that facilitates mass murder. The citizens of this country should be in open revolt. We need change and we need it quickly."
In a tweet, Essex Police said: "We are on scene of disruption on the #M25.
"We received a call at 8am that a number of people were blocking the road between Junction 28 and 29.
"Officers are already on scene and the southbound track has been reopened.
"We’ll bring you more information when we have it."