The climate activists, who plan to grind the motorway to a halt for the rest of the working week, announced the news on Wednesday morning.
Just Stop Oil claim people are "expected to die" after the UK saw record temperatures this week.
Protesters climbed motorway signs and caused disruption. One group of activists at junction 10 eastbound near Cobham, Surrey, said they will climb the gantries that support overhead information signs.
The protest caused nine-mile tailbacks on the M25 anti-clockwise, with the National Highways website warning of delays of more than an hour.
The M25 is one of the busiest motorways in the UK
Steve Parsons
Just Stop Oil activists clash with police
ANDREW BOYERS
A travel alert said: “There are delays of over an hour on the M25 anti-clockwise in Surrey due to an ongoing Surrey Police-led incident within J10 (for A3, Guildford). 3 lanes (of 4) are currently closed.
“If this incident impacts on your planned route, please allow extra journey time as there is over 9 miles of congestion on approach. Plan ahead, you may wish to re-route or even delay your journey.”
Surrey Police said officers were dealing with the protest.
A statement said: “We’re dealing with a protest on the M25 just past junction 10 of the anti-clockwise carriageway this morning. Three lanes are currently closed.
“We will keep you informed of further updates. In the meantime please avoid the area.”
The environmental protest group said more members are on their way, as part of planned action between Wednesday and Friday.
They demand that the Government "makes a statement that it will immediately halt all future licensing and consents for the exploration, development and production of fossil fuels in the UK".
A statement on the protest group's website reads: "This is without doubt the most important moment in UK history.
"All time temperature records are being obliterated, thousands of people are expected to die from heatstroke and the liars and plotters who are vying to lead us are too busy fighting among themselves to even care."
The group plan to march on Westminster over the weekend as they work "in coalition with other groups to escalate civil resistance to force a Government response".
The statement adds: "We are therefore declaring the M25 a site of civil resistance against our criminal Government.
"We ask that no one travels on this motorway from Wednesday to Friday this week as we will be blocking the highway.
"We fully acknowledge the cost and disruption this will cause to the public and ask that they take their demands for compensation to the Government which has caused this unprecedented threat to our lives and liberties."