Insulate Britain said more than 40 of its supporters in two groups have blocked the A20 road.
The offshoot of Extinction Rebellion caused chaos on the M25 motorway five times in the past fortnight, but were threatened with imprisonment if they returned to the motorway.
A spokesperson for the group, which wants the Government to insulate and retrofit homes across the UK to cut climate emissions, said: "We are sorry for the disruption that we are causing.
"It seems to be the only way to keep the issue of insulation on the agenda and to draw attention to how poorly insulated homes are causing ill health, misery and early death for many thousands of people. We are failing the country’s cold hungry families and the elderly and placing an enormous burden on the NHS.
"We are blocking Dover this morning to highlight that fuel poverty is killing people in Dover and across the UK. We need a Churchillian response: we must tell the truth about the urgent horror of the climate emergency.
“Change at the necessary speed and scale requires economic disruption. We wish it wasn’t true, but it is. It’s why the 2000 fuel protests got a u-turn in policy and gave (Tony) Blair his biggest challenge as prime minister.”