Climate activists inside five large containers have blocked the entrances to BP’s head office in central London.
The Greenpeace protesters used cranes to transport the heavy boxes into place at St James’s Square in the early hours of the morning.
Other campaigners abseiled down the side of the building to block windows and display banners.
Greenpeace says those inside the containers have enough food and water to last them for several days.
The aim is to keep BP’s headquarters closed “for at least the whole of this AGM week”, Greenpeace says. BP’s annual general meeting is set to take place in Aberdeen on Tuesday.
Greenpeace said it was carrying out the action to call on BP to end exploration for oil and gas, and only invest in renewable energy.
One campaigner, called Morton, told the BBC the group was planning a “long-term occupation of BP’s headquarters”.
Officers from the Met Police are in St James’s Square. No arrests have been made.
In a statement, BP said: “We welcome discussion, debate, even peaceful protest on the important matter of how we must all work together to address the climate challenge, but impeding safe entry and exit from an office building in this way is dangerous and clearly a matter for the police to resolve as swiftly as possible.”
A company employee said staff had not been told what was happening.
“I’m thinking to go home because it will take the police a while to get the protesters abseiling off the building,” the staff member said.