At least 30 people have been arrested after climate protestors blocked several M25 slip roads, causing delays.
Police warned motorists to avoid parts of the motorway after demonstrators sat on the roads during rush hour.
Essex Police arrested 12 people, while Hertfordshire Police said 18 people had been arrested.
Surrey Police said “a number of people” had been arrested, but did not confirm an exact total.
Protestors for Insulate Britain blocked the slip roads at a number of junctions just after 8am on Monday – including junction 3 from Swanley, junction 6 in Godstone, junction 14 for Heathrow, junction 20 for Hemel Hempstead, and junction 31 for Lakeside.
A number of the roads have since reopened, but are experiencing residual delays.
Superintendent of Hertfordshire Police Adam Willmot said: “We were called to reports of a protest at around 8am today and quickly mobilised resources to the scene.
“The M25 was closed westbound at junction 20, resulting in congestion and delays on both the M25 and the A41.
“Protestors ignored police requests to move location, so we took robust action to enable roads to be reopened and to remove the protestors causing obstructions.
“Our officers dealt with the disruption as quickly as possible, reopening routes for local businesses and residents.
“The road closures were lifted by 10.20am.”
Chief Inspector of Essex Police Paul Austin added: “This incident caused significant disruption and our officers worked to resolve it as quickly and safely as possible.
“I want to thank the drivers affected for their patience and understanding.”
One motorist told Sky News there were around 2.5 miles of queueing clockwise on the M25 at junction 14 this morning.
Footage shared on social media showed angry drivers attempting to drag the protestors from the road.
Video also showed protestors moving out the way to allow an ambulance to pass through.
Surrey Police tweeted: “We’re currently overseeing a protest at J6 M25 reported just before 8am.
“[A number] of protesters from Insulate Britain are at the location, slip road closed+diversion being set up. No arrests made at this stage. Pls avoid the area for the time being.”
Kent Police added: “Motorists attempting to join the M25 from Swanley, at junction 3, may encounter delays due to an obstruction caused by a group of people.
“Our officers are at the scene and engaging with those involved. Traffic is moving and being diverted to join the M25 at junction 4.”
Insulate Britain is a campaign group launched by members of Extinction Rebellion who want the government to reduce home heating emissions to zero.
The protest group said today’s disruption was “just the start”, adding: “Actions will continue until the government makes a meaningful commitment to insulate Britain’s 29 million leaky homes, some of the oldest and most inefficient in Europe.”
However, the AA said the protest “backfires” in terms of its environmental impact.
Edmund King, AA president, said: “This action is not only incredibly dangerous in potentially putting lives at risk, but it also backfires in environmental terms by causing more delays and more vehicle emissions.”