Environmental protesters have sprayed fake blood over the Treasury building in central London.
The Extinction Rebellion demonstrators parked an old fire engine outside the 100-year-old building and dumped 1,800 litres of coloured water.
But the activists lost control of the hose and the red liquid was sprayed across the road.
Three men and a woman have been held on suspicion of criminal damage.
Protesters say they are “highlighting the inconsistency between the UK government’s insistence that the UK is a world leader in tackling climate breakdown and the vast sums it pours into fossil fuel exploration and carbon-intensive projects”.
Phil Kingston, an 83-year-old activist, said: “I fight with all my being for my four grandchildren in this situation of existential danger.”
A Metropolitan Police spokesman said there are no reported injuries and local road closures are in place.