Extinction Rebellion supporters have superglued themselves to the Speaker's Chair inside the House of Commons.
The activist organisation posted a photo of five people inside the chamber, with three holding hands as they stand around the famous seat.
Two others flanking them brandished a sign saying 'Let the people decide' and 'Citizens Assembly now', with all wearing T-shirts reading 'Let The People Decide'.
The group said in a statement on its website the incident is part of a 50 person protest, which also includes others gathered outside Parliament under a large banner.
It had been placed there by an XR member who had climbed up scaffolding around Big Ben and read 'Let The People Decide- Citizens’ Assemblies Now'.
Two activists chained themselves to the railings.
The demonstration, which started around midday, calls for a citizens' assembly on climate and costs to "break the deadlock on Westminster corruption".
XR says those who got into the chamber were part of a pre-booked tour group.
Protester Joe Short, from Bristol, said: "I have locked myself to the railings outside the Houses of Parliament.
"It is part of a bigger protest which is about climate inaction but it is also about a citizens' assembly which is an alternative form of democracy which could be much more effective at dealing with problems like climate change."
Asked if he was uncomfortable chained to the railings and surrounded by police officers, Mr Short said: "At the end of the day, in the scheme of things, it is not such an uncomfortable position."
The image from inside the chamber was shared on the group's Twitter account alongside a caption stating: "Extinction Rebellion supporters have superglued around the Speakers Chair inside the commons chamber.
"Right now inside Parliament a speech is being read out demanding a Citizens' Assembly Now: 'We are in crisis. We can not afford to carry on like this...'"
Parliament is currently on recess for the summer.
XR said the demonstration is part of the first phase of its September plans.
Those who infiltrated the chamber also took it in turns to read a speech "pointing to the need for a Citizens' Assembly to cut through the corruption deep in the heart of Westminster".
The speech read out 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 can not 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.
"We need a new way of making decisions, where more voices are heard, not just those at the top. We need the true diversity of the country to be represented.
“We need a Citizens’ Assembly, now. Citizens’ Assemblies empower ordinary people to make decisions that benefit everyone. Decisions that can get us out of this mess and make life better, safer, fairer for all of us.”
The demonstration comes ahead of a new Prime Minister - either Liz Truss or Rishi Sunak - being chosen next week to replace the departing Boris Johnson.
XR opposed the selection process as being down to a "fraction of the country" - referring to the fact the winner will be selected by Conservative Party members only.
It went on to refer to the UK "suffering from a cost of living scandal meaning millions won’t be able to pay their bills this winter".
Both issues it said has left "faith in politics is at an all time low".
Its statement added: "There is an urgent need to upgrade our political system to allow more representation and give ordinary people a say over the major crises facing us.
"In July this year the high court ruled that the UK Government’s pathway to net zero is unlawful because it is so lacking in detail it’s not even possible to hold them to account on it.
"Recent polling by Ipsos found that eight out of ten people in the UK are concerned about the climate crisis and over 52% percent think the government’s plan to get net zero by 2050 is too late, that’s around 35 million people who think the government’s plan isn’t good enough.
"Yet both candidates for PM have said they plan to increase production of new fossil fuels.
"Our current politics is too focused on the short termism of the election cycle to tackle the major issues of today, like widespread inequality and the climate and ecological emergency.
"A citizens’ assembly on climate and costs would break the deadlock on Westminster corruption, allow more people to be represented, and restore trust in politics."
Alanna Byrne of Extinction Rebellion, said: "It is possible to change things and update politics so it really represents ordinary people.
"Independent citizens’ assemblies can show that those blocking progress in Westminster have no democratic mandate to continue destroying the environment and give power back to people.
"Selected like a jury and supported with independent, expert knowledge, this is true democracy that reflects the diversity of the population.
“But to create a new, fairer politics will require first thousands, then millions of us. It will require sustained culture-shifting civil disobedience, until we become impossible to ignore.
"Then, when there’s enough of us, positive change will become inevitable."
XR says today's demonstration is the "opening act" of its plans for the month which will launch for a "5 phase plan" to bring 100,000 people "onto the streets in civil resistance" in Spring 2023.
On Friday, September 9 a Paint the Streets event is due to take place to round off phase one.
Phase two will see supporters gather the following day at Marble Arch for three days of "deliberative democracy, community building & resistance in an undisclosed, disruptive green space".
The next phase sees Rebellion Buses touring the country for four weeks hosting People's Assemblies.
Phase four takes place on October 14 and invites people back to Westminster for "disruptive nonviolent civil disobedience to join forces with other groups organising around the cost of living scandal".
The final phase centres on "major government disruption over a prolonged period".
It aims to "empower an independent Citizens’ Assembly on climate and ecological justice".
The group adds: "Integrated with our methodical mobilisation campaign, Project 3.5, we will pledge to sign up 100k people by Spring 2023 to come back to London for major nonviolent civil resistance to win on our demands."
A House of Commons spokesman said: "We are aware of an incident on the parliamentary estate and are currently dealing with the situation as a matter of urgency."
