Eight people have been arrested after climate activists glued themselves together around the Speaker's chair in the House of Commons.
The protestors, from Extinction Rebellion UK, were on a guided tour of Parliament when they took the action, a spokeswoman said.
MPs are currently on their summer break, and are due to return next week.
The Met Police said it had launched an investigation into the "full circumstances of the incident".
The protesters read out a speech demanding a "citizens' assembly" on climate issues, the group said.
Other demonstrators were pictured draping banners outside Parliament.
Extinction Rebellion spokeswoman Nuala Lam told BBC News around 50 people had taken part in action in and around the Palace of Westminster.
She added the activists were calling for a "democratic system" that allows "ordinary people to be consulted" on climate change issue.
"I understand there is is some singing going on, and I think that security and police are there," she added.
Following the arrests, the Met Police said in a statement: "Four protesters who were present in the chambers and had glued themselves to each other were de-bonded and arrested.
"None of these protesters were glued to the Speaker's chair and there has been no damage to the Speaker's chair.
Two others - one who had climbed onto scaffolding outside Parliament and another who had glued themself to the pavement inside parliamentary premises - were also arrested, the force said.
Another two protesters who had locked themselves to the Carriage Gates were removed and also arrested.
A Met spokesperson said: "A police investigation into the incident is now taking place in close liaison with our parliamentary security colleagues to establish the full circumstances of the incident."
Earlier, 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".
Commons Speaker Sir Lindsay Hoyle added there would also be an investigation by parliamentary security.
He said: "It is a real shame that those visitors who made arrangements to join tours of the Palace of Westminster today had their visit disrupted and cancelled."
Sir Lindsay added that tours would still take place on Saturday and said he would provide an update to the Commons at the start of business on Monday.
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