Extinction Rebellion protesters have superglued themselves to the Speaker’s chair in the House of Commons and padlocked their necks to the railings outside.
The campaign group said around 50 of its supporters “took non-violent action” demanding a citizens’ assembly to discuss the climate crisis.
Five protesters were pictured in the Commons chamber, with some holding signs which read “let the people decide” and “citizens’ assembly now”.
They are understood to have entered the building after booking an official tour, before breaking away from the group to stage the protest.
One demonstrator told the PA news agency that activists had smuggled banners into the Commons in a “bag within a bag”.
Two protesters could also be seen with locks around their necks and connected to the front gates of the Houses of Parliament.
Banners have also been dropped across The Palace of Westminster building.
In a tweet, Extinction Rebellion said: “The protest points to the need for a citizens’ assembly to cut through the corruption in Westminster, update politics & let the people decide.”
A citizens’ assembly is a group of members of the public brought together to deliberate on policy issues.
Emergency vehicles from the police, fire and ambulance services arrived at the scene shortly after the protest started at 12pm.
Police officers could be seen surrounding the demonstrators and had earlier led a singing crowd of them out of the parliamentary estate.
A police helicopter was also seen flying over the building. .
Parliament is currently on its summer recess until Monday, when it will be confirmed whether Liz Truss or Rishi Sunak has won the Tory leadership race.
A speech read out by activists in the chamber said: “We are in crisis. And what goes on in this chamber every day makes a joke out of us all. We cannot afford to carry on like this.
“It is possible to act on climate and costs in a way that is fair and supports everyone. But our political system is too out of date and out of touch to see beyond the next election cycle and do what needs to be done.”
A House of Commons spokesperson said: “We are aware of an incident on the Parliamentary Estate and are currently dealing with the situation as a matter of urgency.”
A Metropolitan Police spokesman said: “The Met is aware of a demonstration by a number protesters at the Palace of Westminster.
“Met Police and parliamentary staff are responding.”