Drivers were involved in a four-way smash on the M25 as Insulate Britain activists blocked several junctions of the busy motorway for their climate change message.
A 50-year-old woman had to be airlifted to hospital after suffering serious injuries in the pile-up.
Surrey Police said 32 Insulate Britain activists had been arrested just after 10am on Wednesday.
The force said: “We are also dealing with a road traffic collision at Junction 9 of the M25 involving four vehicles.
“Police, fire service and ambulances are currently on the scene.”
A police spokeswoman confirmed that all protesters have been arrested and are no longer on the carriage way.
Dozens of activists from Insulate Britain, which is demanding Government action on home insulation, stopped traffic at two sections of Britain's busiest motorway on Wednesday for the second time in three days.
They targeted Junction 23 for South Mimms in Hertfordshire and the main carriageway from Junction 8 at Reigate to Junction 9 at Leatherhead, both in Surrey, at around 8am.
Demonstrators sat on the road, while stranded motorists beeped their horns.
Videos posted on social media show angry drivers remonstrating with the activists.
President of motoring insurance group AA, Edmund King, said that during the last Insulate Britain blockade, a thermal insulation engineer was prevented from going to work after being stuck in the traffic queue.
On Wednesday he said: "Whilst most people understand the need to take action on climate change, these motorway blockade tactics are just backfiring as they are alienating the working public stuck in this chaos and subsequently pumping out more emissions.
"It is somewhat ironic that thermal insulation engineers trying to insulate Britain were stuck in the congestion caused by Insulate Britain.
"Essential deliveries, emergency services, people missing hospital and business appointments are all hit by these blockades.
"Hopefully the police will take immediate action to unblock the arteries of Britain."
Insulate Britain said 89 of its members took part in the action on Wednesday.
It said in a statement: "We demand credible action now.
"Proper jobs for hundreds of thousands of people to start the first real step - to insulate all the homes of this country - which pound for pound gives us the biggest reduction in carbon emissions.
"It is a total no-brainer and yet this Government refuses to get on with the job. This is criminal negligence."
A demonstration by the same group on Monday saw five junctions of the M25 blocked, leading to tens of thousands of drivers being stuck in huge queues of traffic and dozens of arrests.
