Climate protesters demanding government action on home insulation partially blocked several junctions on the M25 motorway near London.
The Insulate Britain protests came after Extinction Rebellion protests snarled central London this month.
Tens of thousands of motorists attempting to use the UK's busiest motorway faced long delays while activists sat on the road holding banners.
National Highways said there was disruption at Junction 3 for Swanley in Kent, Junction 6 for Godstone in Surrey, Junction 14 for Heathrow Terminal 5, Junction 20 for Kings Langley in Hertfordshire, and Junction 31 for Purfleet in Essex.
Essex Police said 12 people were arrested at Junction 31 on suspicion of highways obstruction.
“This incident caused significant disruption and our officers worked to resolve it as quickly and safely as possible," Chief Insp Paul Austin said.
“I want to thank the drivers affected for their patience and understanding.”
Surrey Police said a number of protesters were arrested at Junctions 6 and 14.
Insulate Britain's 'leak homes' warning
Insulate Britain describes itself as “a new group demanding that the government gets on with the job of insulating Britain's homes".
Its website claimed that the disruption on Monday morning was “just the start”, and “action will continue until the government makes a meaningful commitment to insulate Britain's 29 million leaky homes".
One supporter, Liam Norton, a 36-year-old electrician from London, said he was “shocked at the lack of significant action".
“It's a no-brainer. Insulating Britain will reduce emissions, provide hundreds of thousands of jobs and stop our elderly dying in cold homes each winter. So stop messing about, Boris, and get on with the job,” he said.
“As soon as a statement is made that we can trust and is meaningful, we will get off the roads.”
Another activist, Zoe Cohen, 51, from Warrington, Cheshire called on the government to “stop making things worse with all their road building, airport expansion, HS2 white elephant and new oilfields”.
“We can't get to net zero if they don't take responsibility for this. Only the government can make this happen,” she said.
Protests rile AA chief
Edmund King, head of UK breakdown group the AA, was unimpressed by the “incredibly dangerous” protests, which he said were counterproductive environmentally, “by causing more delays and more vehicle emissions”.
“These are some of the busiest sections of the M25 where tens of thousands of drivers will have been affected and has a negative knock-on effect on economic activity,” he said.
National Highways said it was working hard with police and other partners to limit the effects of protests on the road network.
“We will do all we can to keep drivers up to date and on the move,” said Sean Martell, head of service delivery for the south-east.