Activists carrying inflatable sea creatures marched to BP's headquarters in central London in protest as the firm prepared to drill for oil near a coral reef.
The demonstrators, from environmental campaign group Greenpeace, said the work off the coast of South America risked an oil spill that would harm sea life.
Giant inflatable marine life, including jellyfish and starfish, were paraded along The Mall and past Trafalgar Square by activists, who reached the building shorty after 8am on Monday.
The exploratory drilling work, which is due to begin next year near the recently discovered Amazon Reef, risks an oil spill that could be catastrophic for sea life, the campaign group said.
Signs that read: “BP: Back off the Amazon Reef" were held up by those taking part in the colourful procession.
Activists said they intended to remain camped outside BP, in St James's Square, for several hours after marching through the capital’s streets.
Greenpeace said a potential oil spill could “devastate” the Amazon reef’s ecosystem and “potentially wipe out new species.”
A BP spokeswoman said: “We have a commitment to begin drilling operations for an exploration well on this operated block by August 2018.
“Drilling operations will only begin after a drilling licence is granted by the Brazilian federal environment agency.
“We are engaged in the process required to gain this permission, including submitting an EIA for review and approval.”