More than 40 climate protesters have climbed onto a roof at Parliament in the latest breach of security there.The demonstrators, from Greenpeace, have unfurled banners and flags on Westminster Hall stating "change the politics, save the climate".
Police are currently talking with the demonstrators, although there are no signs they are seeking to remove them.
The latest security breach comes as MPs are set to return to Westminster on Monday after the summer recess.
The police say they have not made any arrests yet.
Protesters say they entered the grounds of the Palace of Westminster by using ropes and ladders to scale perimeter fencing before clambering onto the roof of Westminster Hall, the oldest part of the building.
The activists said they intended to remain on the roof all night and ask MPs to sign a climate manifesto on Monday morning.
One, 28-year old Anna Jones, said the protest would remind MPs they "simply haven't done enough to stop climate change and we need to see some action at home".
Greenpeace denied the protest was dangerous and said it wanted to "raise the temperature" about the climate change debate ahead of Parliament's return.
It said it was alarmed at the lack of attention given to the issue by the political parties, particularly Labour and the Conservatives, at their recent conferences.
Executive director John Sauven said leading politicians from all the major parties were failing to come up with concrete plans for green jobs to match their rhetoric on the subject.
"They have got to act more seriously on the issue," he told the BBC. "It is missing from the political agenda and we have got to get it on the political agenda. That is why we are here."
At its conference in Brighton, Labour announced plans for new cycle hubs at 10 mainline stations and a £10m green neighbourhood programme to help communities reduce carbon emissions.
The Conservatives, meanwhile, said that communities which agree to onshore wind farms will be able to keep revenues they generate from business rates for six years.
The Commons Climate Change Select Committee will publish a report on Monday assessing how the UK is doing on reducing emissions, meeting carbon budgets and building a low-carbon economy.
Security in Parliament was increased in 2004 after some high-profile breaches but questions have continued to be raised about protection levels.
Last year, activists from Plane Stupid occupied the roof for three hours in protest against Heathrow expansion, resulting in five activists being found guilty of trespass.
Incidents in 2004 included a group of campaigners from Greenpeace scaling Big Ben's clock tower.
Pro-hunt protesters stormed the Commons chamber in the same year, while campaigners from Fathers 4 Justice threw purple powder at Tony Blair during prime minister's question time.