More than 20 environmental campaigners remained on the roof of the Palace of Westminster today in a protest over climate change, while another 20 were being held by police.
The Greenpeace activists hope to greet politicians as they arrive for the start of Parliament with a 12-point manifesto calling for zero carbon emissions by 2030, a stop to airport expansion, more wind power and new pollution taxes.
The group is looking to upcoming climate talks in Copenhagen as the ideal opportunity to address the climate problem.
The demonstrators climbed on to the roof, ramparts and a turret on top of Westminster Hall yesterday, unfurling yellow banners which read: "Change the politics, save the climate."
They said there was little resistance from police when they moved quickly to prop ladders against the wall and climb on to the roof.
After four and three quarter hours on the roof, 20 protesters climbed down one by one using a ladder and safety harness and were arrested for trespassing on a "protected site".
A Metropolitan Police spokeswoman said all 20 remained in custody.
Perched high on the roof of Westminster Hall, full-time Greenpeace activist Anna Jones, 28, from Headingley in Leeds, said this morning: "We can't stress enough how important this is. The clock is ticking - scientists are telling us we have just a few years left to act to save the planet.
"Now is the time to start putting policies in place to make sure we are on the right trajectory to peak our emissions and bring them down. But instead we have a government that is prepared to lock us into high carbon projects like the third runway at Heathrow."
She said direct action was necessary and effective.
"Climate change is the biggest challenge we are facing. It takes action like this to make the politicians wake up.
"They are letting us down and letting down citizens around the world."
Ms Jones added that there had been very little police presence as activists quickly scaled the building.
She said the protesters had managed to get some sleep on the roof overnight and had even spotted shooting stars.
"We're planning to stay here for the day. It's quite cold up here but we have had hot meals and are cosy huddled up under one of the big banners," she said.
Greenpeace executive director John Sauven said: "We've got to raise the temperature of the debate because we are really running out of time.
"We are at a minute to midnight and there is so little time left but so much to do.
"Parliament is opening (today) and there is an election looming so this is a golden opportunity for the political parties to really think about the future and what future generations will face."
Speaking from the roof, Brikesh Singh, 29, from Bangalore in India, said: "This building is considered as the mother of all parliaments and the UK is one of the leading developed countries.
"It has to take leadership in solving the climate crisis so that developing countries can follow its lead."
The ease with which protesters gained access to the site raised concerns over security.
Senior Lib Dem MP Norman Baker said: "There is understandable concern about the need to move faster and more effectively to tackle climate change, and Greenpeace are right to seek to protest in a peaceful way.
"But I would have preferred it if they had not done it by breaching the House of Parliament.
"It shows that things are not as they should be in security terms.
"The House authorities really need to sort this out."
A spokeswoman from the Department for Energy and Climate Change said: "Public demonstration to highlight the need for action on climate change by all within society is welcome, but it must be conducted in a way that respects the rule of law.
"We have a comprehensive plan to transform our economy and society, by investing in green jobs, cleaning up our energy supplies and making our homes energy efficient.
"This week we host vital talks to accelerate the development of clean coal and to progress the Copenhagen deal. It's our domestic record that has given us the credibility we need to press hard for an ambitious global climate deal."
A House of Commons spokeswoman said: "We can confirm that 45 protesters climbed up the wall adjacent to the Cromwell Green building.
"Police have the situation contained. At no time did the protesters have any access to the building, nor did they seek it."
As the sun glinted on Big Ben, the campaigners proudly held up their fluorescent yellow banners.
A few cars sounded their horns in support of the rooftop protest amid the rush-hour traffic.
On the pavement below, Greenpeace campaigner Robin Oakley said morale was high and the group planned to stay as long as possible.
He said: "Our volunteers have spent the night on the roof and have woken up to a dawn where they are determined to get the message across to MPs who are back today.
"This morning the Committee On Climate Change - the Government's expert advisers - are saying we need a step change in Britain's efforts to cut emissions and the volunteers on the roof are determined that that message gets across.
"Until Britain puts climate change at the heart of its economic policy and industrial strategy, we won't get the clean energy, green jobs and CO2 cuts that we need both for Britain and to rebuild trust in international negotiations.
"It is a powerful thing to wake up at the heart of Britain's democracy with an urgent and vital message for Britain's politicians.
"It is a message for everyone that works in this place and it is extremely relevant today as the MPs return from the summer recess with just a few weeks to go before the international climate summit in Copenhagen."
Shortly before 8am the activists began using ropes to climb from a turret up to the higher and larger roof of the Great Hall.