Hundreds of people are staging "protest of one" road blocks on Saturday, which sees individuals sitting alone in busy roads wearing signs with messages about their fears for the future.
The protest began at 11am – two years exactly since Parliament declared a climate emergency.
Morgan Trowland, 38, glued himself to London’s Tower Bridge, prompting City of London Police to close it to southbound traffic.
Mr Trowland, a civil engineer from Hackney, east London, said: "I’m terrified that billions will die because of the climate crisis, humans and non-humans.
"I’m freaking out that many people are accepting this, or feel powerless to change the course. I want to show onlookers that we each have phenomenal power."
Mr Trowland was later arrested and charged with obstruction of a highway.
Another man blocked Blackfriars Bridge holding a placard reading: "I’m terrified billions will starve while our government refuses to act on the climate crisis."
Bing Jones, a 68-year-old retired NHS doctor, blocked a road in Sheffield.
He said: "I’m terrified that our climate targets are empty promises.
"We have targets for 2050 but we are still increasing emissions today.
"The government has made inexcusable Covid mistakes. I’m terrified that they will be even worse on the climate crisis."
He added: "I am desperate. I write, lobby and support green politics but see nothing real happening.
"So I am sitting in the road today with a notice saying I am terrified. I am sorry to disrupt ordinary people but I do not know what else to do."
Nurse Andrea Muntiu, 36, blocked traffic in her home town of Ipswich.
She said: “People may wonder what a nurse is doing sitting in the road with a placard.
"They may wonder what impact this will have on my career or whether I’ll get in trouble with the police.
“I am aware of the risks but I feel I have a moral duty to warn people about the catastrophic consequences of the climate crisis.”
She added: “We can still change things, we can demand the government acts now and safeguards what we have.”
Lynda Duffill, a 53-year-old mental health trainer and volunteer manager from north Pembrokeshire who sat in the road in Haverfordwest, said: "I fear for the next generation’s future. I work with young people and have two young adult children myself.
"They deserve a life free from the difficulties that climate breakdown could bring."
Protests took place in towns and cities across the country, including Nottingham, Birmingham, Oxford, Bradford, Newcastle and Swansea.
XR said in a statement that despite its carbon-cutting targets - with Boris Johnson recently announcing plans to cut emissions by 78% by 2035 - the UK government is moving too slowly.
It cited multiple reports finding the UK will miss its targets unless ministers take rapid action.
XR spokesperson Gully Bujak said the government was good at generating positive headlines but accused it of failing to have a plan to meet its goals.
Ms Bujak said: "The UN People’s Climate Survey found that 81% of people in the UK believe that we’re in a global emergency.
"The government needs to commission a national Citizens’ Assembly on climate and ecological justice, because even if politicians are scared to face the truth, ordinary people are ready for action.”
Saturday’s protests coincided with a number of demonstrations against the Police, Crime, Sentencing and Courts Bill.
The proposed legislation would give police in England and Wales more powers to impose conditions on non-violent protests – including those deemed too noisy or a nuisance, with those convicted liable to fines or jail terms.
A Home Office spokesperson said: "The right to protest is a cornerstone of our democracy, but over recent years we have seen an increase in the use of disruptive and dangerous tactics."
He added: "These new measures will not stop people from carrying out their civic right to protest and be heard, but will prevent large scale disruption – enabling the silent majority to get on with their lives."