Dozens of activists have coated themselves in plaster and are trying to occupy the British Museum overnight in a bid to pressure the institution to cut ties with oil corporation BP.

About 60 protesters were taking part in the defiant act of impromptu sculpture making as the museum in London attempted to close its doors at 5pm on Saturday.

The action, entitled Monument, is the first of its kind to be conducted by the theatrical protest group BP or not BP?

“We are imagining a world in which the British Museum has stopped celebrating those causing the climate crisis and is instead allying itself with those who have, currently are, and will be putting their bodies on the line in the fight for climate justice,” said a plaque describing the protest.

The civil disobedience campaign began on Friday night when protesters dressed as ancient Greek warriors smuggled a 13-foot Trojan horse into the museum’s foyer. The protest was in response to an exhibition, Troy: Myth and Reality, which is described as “supported by BP”.

Two guards slept inside its belly overnight, despite a downpour at around 4am, to prevent it being removed.

The group estimated that as many as 1,500 supporters took part in the action on Saturday. Protesters occupied 11 of the museum’s rooms, which became host to spoken word performances, singalongs and talks by campaigners from West Papua and Senegal.

Origami swans were scattered throughout the museum adorned with messages including: “Choose sponsors who care about our future”.

Protesters finished the day by tearing apart paper versions of BP’s logo near the museum’s main entrance.

“We feel that the museum’s attitudes towards climate change and colonialism are not what they should be in the 21st century,” said the group’s Jess Worth.

Suzanne Savage, from Malvern, Worcestershire, was among a gathering of fee-paying British Museum members who unfurled a banner emblazoned with the words “BP must fall”.

She said: “I have been a longstanding member of the British Museum. I very much support the art but we do not want this noble institution’s reputation being sullied by sponsorship from a dirty oil company, which is one of the biggest polluters in the world.”

Hartwig Fischer, the director of the British Museum, said: “The museum is a public space where people can come to debate and we respect other people’s right to express their views.

“We share the concerns for the challenges that we all face together as a result of climate change. We address these issues in an innovative way through significant exhibitions and public programming.

“The British Museum offers for millions of people an extraordinary opportunity to engage with the cultures and histories of humankind. Without external support and sponsorship this would not be possible.

“Removing this opportunity from the public is not a contribution to solving the climate crisis.”

The protest is the latest move in activists’ campaign to end fossil fuel sponsorship of the UK’s leading cultural venues.

In October, the Royal Shakespeare Company ditched its sponsorship deal with BP, after a campaign from artists, environmentalists and members of the public.