Fourteen activists from Greenpeace UK, equipped with ropes and hard hats, scaled the pillars in front of the museum's main entrance.
They were protesting BP's sponsorship of the British Museum's latest exhibition, "Sunken Cities
," which opens to the public on Thursday.
The exhibition traces the discovery of two previously unknown Egyptian cities which sunk in the Mediterranean Sea more than 1,000 years ago.
The activists unfurled seven posters, renaming the exhibition "Sinking Cities" and depicting places affected by extreme climate change: New Orleans, badly hit by Hurricane Katrina, the Maldive Islands, Manila in the Philippines, as well as British coastal towns that would be submerged if sea levels continue to rise.
Greenpeace UK says in a statement it is "taking a stand because of the irony of an oil company sponsoring an exhibition whose name practically spells out impacts of climate change. What were they thinking?"
The British Museum later tweeted that it was closing temporarily.
According to a Metropolitan Police spokesperson, officers were called to the scene but there are no reports of arrests.
The museum's closure drew crowds around the area and provoked some Twitter reaction:
The museum reopened on Thursday afternoon with the main courtyard cleared of protesters and police.
A British museum spokesperson said the decision to close the museum was taken to ensure the safety of the visitors.
"We would like to thank visitors for their patience. The British Museum is grateful to BP for their long-term support which has allowed the museum to bring world cultures to a global audience through hugely popular exhibitions and their associated public programs," the spokesperson said.
A series of protests
This is the latest in a series of protests surrounding BP's partnership with the British Museum.
The protests culminated this week with four actions targeting the "Sunken Cities" exhibition. The other campaigns were staged by activist organizations, Art not Oil and BP or not BP.
Elena Polisano, a Greenpeace UK campaigner present at Thursday's protest, told CNN that the problem of sinking cities "is not in the past, it's very much in the present and could be in the future as well if BP continues its business. It's been exacerbated by climate change
and that's BP's business."
She added that the oil company is only "using the British Museum to clean up its image."
"We know that the deal [between the British Museum and BP] is under negotiations at the moment and we hope that the new director will drop BP's sponsorship."
The British Museum's new director, Hartwig Fischer, was appointed in April this year, replacing Neil MacGregor.
Greenpeace UK is hoping that the British Museum will follow in the footsteps of the Tate gallery in London which has not renewed a BP partnership for next year.
Also present at Thursday's protests was Chris Garrard, a campaigner with BP or not BP, another organization protesting BP'S arts partnerships.
"BP get a legitimacy they don't deserve from showing their logos [at the British Museum], and the irony is that the exhibition is about Egypt, and Egypt is one of the countries most vulnerable to climate change and rising sea levels."
Garrard wrote a report
drawing on Freedom of Information requests which revealed "the multiple ways in which BP has interfered in the running of the museums and galleries it sponsors."
The report also says that "British Museum staff describe BP as 'extremely demanding' and 'bullying.'"
A British Museum spokesperson says the museum "does not recognize the sentiment behind that statement."
A BP spokesperson added that "BP has a long history as a major supporter of arts and culture in the UK and we are proud to have partnered the British Museum for 20 years, supporting significant exhibitions such as the new Sunken Cities exhibition."