Animal rights protesters have been filmed scaling the walls of the Home Office HQ in Westminster.
The Animal Rebellion activists are demanding the world switches to a "fully plant-based diet" ahead of the COP26 climate summit in Glasgow.
The Home Office building also houses the offices of the Department for Environment, Food and Rural Affairs (Defra).
Four demonstrators have vowed to remain suspended from the building until Boris Johnson commits to ending subsidies for the meat and dairy industry.
They will stay in hammocks until their demands are met, Animal Rebellion said.
Fire engines and ambulances have rushed to the scene in Marsham Street and police are 'engaging' with the protesters.
No one has been arrested so far.
Activist Claudia Penna Rojas told PA: "We are here in the lead-up to Cop26 demanding that the Government takes drastic action by transitioning to a plant-based food system as the science tells us we need to."
She acknowledged that "not everyone will be willing to give up meat immediately" but added: "What we are asking is for the Government to be responsible, to stop subsidising animal agriculture - dairy, meat - and begin subsidising more ethical, sustainable ways of farming."
Animal Rebellion spokesman Nathan McGovern said: "The UK Government is simultaneously saying that it is a world leader in climate change whilst propping up the unsustainable and unprofitable meat and dairy industry, one that emits disproportionate amounts of greenhouse gases.
“This hypocrisy has to end – we need to defund meat and subsidise plant-based alternatives instead."
Sky News presenter Kay Burley tweeted a video of the protesters hanging on the outside wall of the building.
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A spokesperson for the Metropolitan Police said: "Police were called at 6.09am on Tuesday, 26 October following reports that a number of protesters had scaled a government building in Marsham Street SW1.
"Officers are on scene with the London Fire Brigade. There have been no arrests at this time."
Animal Rebellion said the UK government spends at least £1.5 billion a year on subsidising livestock farming.
The group is calling on the Prime Minister to subsidise plant-based foods instead.
The protest comes ahead of world leaders arriving in Glasgow for the 26th United Nations Climate Change Conference, which starts later this week.
Reacting to the demonstration, a UK Government spokesman said: “We are now only days away from COP26 in Glasgow where, building on commitments made at the G7 summit in Cornwall, we will continue to lead by example to drive global action on tackling climate change and reducing emissions.”
