A Extinction Rebellion demonstrator, thought to be from Ambleside in Cumbria, has made a "free solo climb" of Big Ben.
A spokesman for the group said the climber was 43-year-old tree surgeon Ben Atkinson.
Mr Atkinson displayed an Extinction Rebellion banner from the tower - currently under maintenance and surrounded by scaffolding - to "highlight Government inaction on the climate and ecological emergency".
The demonstrator appeared to be mimicking Prime Minister Boris Johnson with a blonde wig and suit.
One sign showed the Extinction Rebellion logo and the words "No pride on a dead planet" another said: "Citizens Assembly".
In a Facebook post, Mr Atkinson wrote: "Gone and done it... Extinction Rebellion. Here to stay."
"We got the declaration of climate emergency last time. Now perhaps, please Boris, you're struggling to navigate Brexit let alone work our way out of the sixth mass extinction event. Please. Pretty please.... let's get on with the citizens assembly. Thank's Boris.
"I know somewhere in there you have compassionate feelings towards something other then your career. You've got Premiership. Now you've got a chance to do something that could last for centuries. A legacy of being the man that turned it all around. Dare you.
"Oh, and thanks Greta. I heard you calling."
A spokeswoman for the Met Police said the force had officers at the scene, but declined to comment on how they were planning on getting Mr Atkinson down.