Direct action group Extinction Rebellion has brought cities to a standstill and vowed to do the same at the UN climate change conference in Glasgow later this month.
In recent weeks, a previously unheard-of offshoot, Insulate Britain, has also caused gridlock on motorways and main roads, sparking scores of arrests and a court injunction.
Prime Minister Boris Johnson on Wednesday called the protesters a "confounded nuisance" and welcomed moves for "new powers to insulate them snugly in prison where they belong".
The government is keen to lead the way on reducing carbon emissions and ensure new binding targets to cut global warming are met at the summit.
But it is also takes its cues from a largely right-wing British press that is increasingly hostile towards the activists and calls them an "eco-mob" and "enviro-idiots".
Both Extinction Rebellion and Insulate Britain have been accused of putting lives in danger with their tactics, which have included protesters gluing themselves to the tarmac and sitting in front of rush-hour traffic.
On Monday, footage showed one desperate driver begging to be let through a protest in south London so she could follow an ambulance carrying her mother to hospital.
- Beards and woolly hats-
When Extinction Rebellion founder Roger Hallam was asked whether he would block an ambulance carrying a dying patient, his reply was simply: "Yes."
But other activists disagree.
"We are heartbroken by all of this. We're not going out there to stop ambulances getting through," said Tim Speers of Insulate Britain.
Speers, 36, from Cornwall, in southwest England, bears little resemblance to the media caricature of an environmentalist -- a bearded, woolly hatted "crusty" as Johnson has called them.
Clean-shaven, fast-talking and a former professional poker player, Speers said he left his old life behind to fight climate change through civil disobedience.
"As soon as they come out with a meaningful statement that they will get on with their job, they will meet their own targets, I will get off the road," he said.
"I cannot sit by while this government completely fails the citizens it is obliged to protect."
Britain has seen many environmental protests in the past, like the ones over infrastructure projects such as a road bypass near Newbury, western England, in the 1990s.
Daniel Hooper, nicknamed "Swampy", was one of the activists who tried to block construction by tunnelling under that site, and he re-emerged earlier this year on another protest in London.
He has been on trial with other campaigners, including the children of a millionaire landowner and publisher, for trying to prevent construction of the HS2 high-speed railway line.
The HS2 Rebellion group spent days in tunnels they secretly dug near the Euston mainline terminal.
On Monday, Speers was outside London's Royal Courts of Justice as more than 100 protesters from the group were served with an injunction against blocking roads.
Some sported beards and woolly hats, but most were drawn from a diverse range of backgrounds, from parents and their children, to the elderly and even members of clergy.
Retired IT consultant Janine Eagling, 60, said that after 30 years of environmental campaigning she joined Insulate Britain because of the need for urgent action.
- 'No tomorrow' -
"We're in a worse position than ever. We're emitting C02 like there's no tomorrow, which, if we carry on like this, then literally there will be no tomorrow," she said.
"It may seem extreme that we're disrupting people in their everyday lives... (but) Insulate Britain has got one simple demand."
Home Secretary Priti Patel on Tuesday announced new measures to deal with Insulate Britain, which wants all British homes to be environmentally efficient, and others.
She said she would not tolerate "eco-warriors trampling over our way of life and draining police resources" as she announced maximum penalties for motorway disruptions and plans to criminalise interference with infrastructure.
Insulate Britain blasted Patel and other ministers as "cowards", warning that blaming campaigners would do more harm than good in the long run.
"Shooting the messenger can never destroy the message: our country is facing the greatest risk ever and our government is failing us", they said.
Confrontation could be on the cards in Glasgow, with a planned rally of 50,000 to 100,000 people during the summit.
Police Scotland, which is deploying some 10,000 officers every day of the two-week meeting, has said it will facilitate peaceful demos and "unlawful protest to a point".
But it has warned action may have to be taken "when the protest starts to impinge on the ability of conference to operate".