Six Greenpeace activists are climbing up the Shard building in London to protest against oil company Shell's plans to drill in the Arctic.
The six women are attempting the scale the tallest building in western Europe after gaining access by climbing from the roof of London Bridge rail station.
The protesters said once they reach the top of the 310m (1,016ft) building they will hang a banner which will capture the "beauty of the Arctic".
A statement on the savetheartic.org website said: "This building - modelled on a shard of ice - sits slap bang in the middle of Shell's three London headquarters. They don't want us talking about their plan to drill in the Arctic. We're here to shout about it from the rooftops."
The charity also tweeted photographs of the woman beginning their ascent in the early hours of the morning using ladders and climbing ropes.
One of the climbers, Victoria Henry, 32, a Canadian living in Hackney, London, wrote in a blog post before the protest: "We'll try to hang a huge art installation 310m up that will make Shell think twice before sending their rigs into the Arctic.
"It's going to be really hard work, it's going to be nerve-shredding for all of us and we may not succeed, but we're going to do everything we can to pull it off.
"Millions of people have called on Shell to get out of the Arctic but they're still trying to drill there anyway.
"If we reach the top we'll be able to see all three of Shell's London offices below us, meaning they'll be able to see us.
"Maybe then they'll stop ignoring the movement ranged against them."
A spokesman for the Metropolitan Police said: "We were called at twenty past four this morning. We have six protesters attempting to scale the outside of the Shard.
"We have officers down there, monitoring the situation. The event is still ongoing."
The 87-storey building was officially opened in February.
A Shard spokesman said: "The Shard is being used by protesters as part of a campaign.
"Our primary focus is on the safety of the protesters and the workers and visitors to the building.
"We are working with the relevant authorities to try to ensure the safety of those concerned."
Shell said: "We respect the right of individuals and organisations to engage in a free and frank exchange of views about our operations.
"Recognising the right of individuals to express their point of view, we only ask that they do so with their safety and the safety of others, including Shell personnel and customers in mind."
Months of training and secrecy end here. I'm scared but incredibly excited about today #iceclimb
— Victoria Henry (@victohenry) July 11, 2013
Greenpeace activists previously forced the closure of dozens of Shell petrol stations in London and Edinburgh in protest at the company's plans to drill for oil in the Arctic.
The group hijacked more than 45 stations and turned off emergency switches - stopping the flow of petrol to the pumps and effectively shutting the stations down.