An Extinction Rebellion demonstrator is attempting a “free solo climb” of Big Ben, the group said.
A spokesman for the group said the climber was tree surgeon Ben Atkinson, 43.
He said Mr Atkinson was planning to drop an Extinction Rebellion banner from the tower – currently under maintenance and surrounded by scaffolding – to “highlight Government inaction on the climate and ecological emergency”.
Extinction Rebellion activists, joined by celebrities including Steve Coogan, have led a protest to Downing Street and blocked roads in central London on the penultimate day of a fortnight of demonstrations.
The climate change activists brought traffic to a standstill with a large bamboo structure at Oxford Circus in London’s West End, as part of the penultimate day of their “Autumn Uprising” campaign.
On Friday the protesters targeted what is a vital transport hub due to its proximity to luxury goods shops, after targeting airports, government departments and London landmarks since October 7.
Hundreds of activists, including actor Steve Coogan, also marched through Westminster demanding the Government takes immediate action on climate change, defying a police ban on the group protesting in London.
The protesters used red washable chalk spray to mark the path of their march with red hand marks, from Whitehall Gardens to six government departments.
Members of the group then staged sit-ins outside the Treasury and the entrance to Downing Street, ahead of the planned end of the action on Saturday at 6pm.
Mr Coogan became the latest celebrity to support the Extinction Rebellion cause, as he joined the demonstrators on the march to Whitehall.
He told the PA news agency: “I wanted to lend my voice to the cause.
“Maybe people who like what I have done and who might not be completely sold on Extinction Rebellion might give it a second chance.
“It’s an important movement. Some people try to characterise it as a fringe movement, but it’s not.
“I like the fact that it was started by young people, but now older people have joined in too.
“Climate change feels like just another issue on the agenda that people are reluctant to pay attention to.
“But people in Extinction Rebellion and people who seriously believe want to make it a bigger issue.”
He added: “Anyone who is against the movement is motivated purely by the bottom line.
“I have always been environmentally aware and environmentally conscious. I just do not want to stand on the sidelines and not participate. It’s easy to sit and not do anything.”
By shortly after 2pm on Friday, 1,768 arrests had been made since the start of the campaign 10 days ago.
The rate of arrests is believed to have accelerated from Monday after the Metropolitan Police imposed an order temporarily banning all Extinction Rebellion protests.
When asked about why she was protesting outside Downing Street despite the police ban, Phyllis Wolff from Dorset said it would be a bigger risk if they did not act.
“It’s a bigger risk for us not to do this,” she told the PA news agency.
“I look at my grandchildren and I think, how can I not do this?”
Dame Vivienne Westwood also joined Extinction Rebellion activists and members of Free West Papua outside the London head office of BP, where they were delivering a report on the company’s activities in West Papua.
Extinction Rebellion was forced to apologise after a controversial demonstration on the London Underground network on Thursday, which prompted significant criticism both outside and within the decentralised protest movement.
After activists targeted a Tube train at Canning Town station, a protester was dragged off the roof of the train by an irritated commuter.
The group apologised for the disruption caused to commuters but said the incident should not create “unnecessary division”.
