Extinction Rebellion activists caged themselves like canaries to block the entrance to an open-cast coalmine in opposition to plans to expand it.
One protester tried to get under a barrier at the Banks Group’s site at Bradley near Dipton, County Durham, but was stopped by guards.
Durham County Council has received more than 5,500 objections to the plan, many on environmental grounds.
The Banks Group has been approached for comment.
The firm has applied for planning permission to extend the mine to extract a further 90,000 tonnes of coal and 20,000 tonnes of fire clay.
About 50 protesters have gone to the site on the first of three days of planned action at the mine.
Four of them are dressed as canaries, whose large wire cages are blocking the mine’s entrance, as campaigners chanted “we are non-violent”.
One, who did not want to be named, said: “If things got unsustainable for life underground the canaries down the mine used to peg it before the miners did.
“Here we are, trying to stop the coalmine because there’s no need for coal.”
The campaigners said that extending the mining operation went against the commitment to phase out coal by 2025 and to become carbon-neutral by 2050.
Durham Police was monitoring the protest and warned drivers the A692 could be affected.
Plans for the mine were first approved in 2015 and extraction of coal began in May 2018.