Environmental campaign group Extinction Rebellion have used a fire engine to spray the front of the Treasury building with fake blood.
The group unfurled a banner urging the government to "stop funding climate death" and said they had fired 1,800 litres of the "blood" at the central London building – although most appeared to have ended up on the street and pavement.
While standing on top of the fire engine, protesters struggled to control the hose as it whipped around on its own accord.
Police later turned up to protect the building and four people were arrested on suspicion of criminal damage.
Campaigners said the fire engine they used is out of commission and the fake blood made with food dye that can be washed off.
Extinction Rebellion said it wanted to highlight the "inconsistency between the UK Government's insistence that the UK is a world leader in tackling climate breakdown, while pouring vast sums of money into fossil exploration and carbon-intensive projects".
Retired grandfather of four Phil Kingston, 83, was of those taking part in the spray-down protest.
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The others included a retired GP, an ex-Buddhist teacher, a musician, and an electrician from south London.
They said their protest was part of the build up to the "International Rebellion" – beginning in four days' time – when they plan to block 12 areas around Westminster for several weeks.
The group's actions at Easter brought parts of the capital to a halt and included stationing a pink boat emblazoned with the words "tell the truth" in the middle of Oxford Circus.