Eight people have been arrested after Extinction Rebellion activists tried to spray fake blood at the Treasury in Westminster from the top of a fire engine.

Dressed in funeral attire stood on the vehicle – which had a banner reading “Stop funding climate death” on its side – the protesters both criticised the UK’s military role in the Middle East and the UK’s investments in companies that cause vast fossil fuel emissions, and called on others to rebel.

They splattered the main HM Treasury building on Horse Guards Road in London for several seconds until they lost control of the hose, drenching a bystander and spraying several fellow activists as 1,800 litres of an organic liquid containing beetroot spurted out wildly across the street.

After about 15 minutes dozens of police officers arrived and arrested four activists, telling them to be careful until they could be safely brought down from the vehicle.

Mark Ovland, one of the protesters, said he was prepared to be jailed for the action. “There needs to be a real wake-up in terms of the money being spent by the government,” he said from the top of the out-of-commission fire engine.

“Decisions being made in this building are going towards a non-future. We’re funding billions in fossil fuel subsidies and carbon-intensive projects, and we just need a rethink otherwise we’re in serious danger.”

After the ladder arrived more than an hour later, the campaigners voluntarily climbed down and were led away by officers. As he was being escorted to a police van, 83-year-old Phil Kingston said the Treasury was doing “dreadful things”.

Cathy Eastburn, another protester, said as she was frogmarched into a police vehicle that her children did not have a future if policies which had “devastating consequences” on the environment continued to be pursued.

“They need to stop funding projects that are causing more global warming and meaning that we don’t have a future for our children,” she said. “All children everywhere do not have a future if we carry on doing this.”

The group said the UK remained the largest fossil fuel subsidiser in the EU and highlighted a recent report by a Commons committee that said the government credit agency, UK Export Finance, was undermining the country’s international climate and development targets.

“UKEF’s support for fossil fuel energy is unacceptably high with the majority of investments in projects in low and middle-income countries,” it read. “This risks locking them into high-carbon dependency for decades to come”

The Metropolitan police said that a total of eight people were arrested on suspicion of criminal damage; five men aged between 34 and 83, and three women aged between 34 and 60. It confirmed that the fire engine was seized and removed after the force was called to the scene just before 10.20am.

It said there were no reported injuries and that local road closures were in place while police dealt with the incident.

A supporter said he “took his hat off” to the activists. “We all need to make sacrifices, whether as an activist or an individual, in order to save our planet and keep it liveable for our children,” said Robert Possnett, who was found guilty of acting in a disorderly manner this year at a public meeting.

However, one bystander criticised the action, in which liquid was sprayed powerfully across a path, saying: “You morons, you put everyone in jeopardy.”

The protest comes before a two-week “international rebellion” starting from Monday. Extinction Rebellion has said it will “peacefully occupy the centres of power and shut them down” until governments act on the climate emergency. “Take two weeks off work,” a leaflet being distributed read.

A Treasury spokesman said: “The UK is a world leader on climate change – having reduced its emissions by 42% between 1990 and 2017, while growing the economy by more than two-thirds.

“In June, we became the first major economy to legislate to end our contribution to global warming by 2050. We will continue to build on this proud record.”