Activists tipped several tonnes of coal at a back entrance but police stopped a truck with another 14 tonnes heading for the main gates.

"At approximately 7:30 am (0730 GMT) on Monday, demonstrators deposited some coal from a tipper truck on Horse Guards Parade at the rear of Downing Street," London's Metropolitan Police said.

"Some of the occupants from the vehicle then got out and left small piles of coal in King Charles Street (another access point).

"The truck has been stopped and the police have spoken to the demonstrators.

"The coal is being cleared away and there are no arrests at this stage."

Downing Street refused to comment.

Greenpeace executive director Stephen Tindale, a former environment advisor to Blair's ministers, said: "We've blockaded Downing Street with coal because Tony Blair has failed on climate change.

"So far all he's done is make speeches.

"Blair's burning more coal than ever, our CO2 (carbon dioxide) emissions have gone up, he's set to miss his own global warming targets and now it seems he's trying to kill off the Kyoto Protocol."

The Kyoto treaty commits signatories to trim their output of six greenhouse gases, especially carbon dioxide, by 2012 compared with 1990 levels.

Blair has made action on climate change one of the themes of Britain's 2005 presidency of the Group of Eight leading industrial powers, alongside African poverty.