Campaigners have blockaded the Department for Transport (DfT) to protest against what they claim are Government efforts to prevent moves to restrict "tar sands" from entering Europe.
Greenpeace said 50 protesters had installed large plywood boards, locks and chains and parked two cars in front of the entrances to the Whitehall department while some campaigners had chained themselves to the doorways and to each other.
The plywood boards installed outside the DfT were painted with a giant "lobbying" handshake and a banner was hung up reading "HM Department for Tar Sands".
The environmental group claims ministers are attempting to undermine European Union (EU) moves to rule tar sands as more polluting than conventional transport fuels, following lobbying from the Canadian government and the oil industry.
Environmentalists oppose tar sands because of their effect on the climate, as the fuel takes far more energy to extract than conventional oil, producing more emissions, as well as their impact on the forests of Alberta.
Under the Fuels Quality Directive, the EU has committed to reducing the emissions from the production of transport fuel used in Europe by 6% by 2020.
Environmental groups said the Commission's proposal to treat tar sands extraction as significantly more polluting than conventional oil production would effectively ban its use in Europe if the bloc is to meet the directive's target.
The Greenpeace campaigners criticised the Government for going to the UN climate talks, which begin in Durban, South Africa, to focus on cutting emissions while "doing everything to scupper" an EU plan to reduce greenhouse gases.
Paul Morozzo, Greenpeace energy campaigner, said: "Extracting oil from tar sands emits on average between three and five times more carbon dioxide than conventional oil drilling.
"David Cameron and Nick Clegg should intervene to ensure oil lobbyists and their allies in the Department for Transport don't manage to derail this key European move away from one of the dirtiest energy sources known to man."