The campaign group said about 50 of its supporters “took nonviolent action” to call for a citizens’ gathering to discuss the climate crisis.
Five demonstrators were pictured in the Commons Chamber, some with signs that read “Let the people decide” and “Citizens’ gathering now”.
They are understood to have entered the building after booking an official tour, before parting ways with the group to protest.
A demonstrator told the PA news agency that activists had smuggled banners into the Commons in “a bag within a bag.”
The two protesters can also be seen with their necks locked and attached to the front gates of the Houses of Parliament.
Banners have also been dropped at the Palace of Westminster Building.
In a tweet, Extinction Rebellion said: “The protests point to the need for a civil assembly at Westminster to end corruption, update politics and get people to decide.”
A citizens’ assembly is a group of members of the public that are brought together to discuss policy issues.
Police, fire brigade and ambulance vehicles reached the spot shortly after the strike started at 12 noon.
Police officers could be seen surrounding the protesters and had previously led the singing crowd outside the parliamentary constituency.
A police helicopter was also seen flying over the building.
Parliament is currently on its summer break 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 everything that happens every day in this chamber makes fun of us all. We cannot afford to proceed like this.”
“It is possible to act on climate and cost in a way that is fair and supports all. But our political system is too old and out of touch to look beyond the next election cycle and do what needs to be done. is out.”
A House of Commons spokesman said: “We are aware of an incident on Parliamentary Estate and are currently viewing the situation as an urgent one.”
A spokesman for the Metropolitan Police said: “The Met is aware of demonstrations by several protesters at the Palace of Westminster.