Greenpeace protesters spent much of the day on Admiralty Arch in central London over claims the government is using illegally logged wood.
The Mall was closed for two hours
Fourteen protesters scaled the building on Wednesday morning but came down peacefully after the evening rush hour.
They unfurled banners accusing the government of "trashing rainforests" during Cabinet Office renovations.
The government said its policy is to use timber from legal, sustainable sources, but it will investigate.
The Mall was closed for two hours soon after the protesters climbed the historic building, just off Trafalgar Square.
Police said the demonstrators gradually came down throughout the day until the last was down by 1830 BST. There were no arrests.
Renovation work is being carried out at Admiralty Arch, which houses parts of the Cabinet Office and the government's Strategy Unit.
Greenpeace said plywood hoardings being used are made from illegally logged timber from the rainforests of Papua New Guinea.
The protesters scaled the building in the early hours
The charity has occupied Cabinet and Home Office sites before in similar protests and recently claimed plywood used during the restoration of Nelson's Column - overlooking Admiralty Arch - was from Papua New Guinea.
Greenpeace boss Stephen Tindale, who is among protesters on top of Admiralty Arch, said the government's timber procurement policy was totally ineffectual.
"Blair has a history of talking up his green credentials, but it's about time he actually took effective action," he said.
"The first thing he needs to do is to ban the import of illegal timber into the UK market place."
Sam Moko, from Papua New Guinea, was expected to deliver a piece of illegal timber to Downing Street as part of the protest.
He said logging companies were chopping down the forests so fast the future of forest-dependent communities were under threat.
In a statement, the government said: "The Cabinet Office policy is to use materials from sustainable sources and we have a contract with a company which requires them to obtain their wood from legal, sustainable sources.
"They have provided certification to show they have done so.
"We are concerned about the Greenpeace allegations and are looking into them."