Six women are illegally climing the 72-storey Shard building in London to raise awareness about oil exploration in the Arctic.
The Greenpeace activists are filming their climb and streaming coverage as it happens.
Greenpeace says, "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... 1 skyscraper. 6 women. No permission. What will you do to save the Arctic?"
Greenpeace posted this picture of London commuters seemingly hypnotised by what they are seeing above
Police were called at 4.20am when the activists, who are from the UK, Canada, Sweden, Poland, Holland and Belgium, started their climb.
Greenpeace maintains the activists carried out a safety assessment beforehand.
The women were able to access the Shard from the roof of nearby London Bridge station, after months of planning and training.
The campaigners said they chose the Shard because of its proximity to oil company Shell's three London offices.
"Shell is leading oil companies' drive into the Arctic, investing billions in its Alaskan and Russian drilling programmes," Greenpeace said in a statement.
The lead climbers are understood to be 'free climbing' (scaling the building without assistance) but are fixing safety ropes as they make their way to the summit.
Greenpeace are streaming live video of the attempt from a camera worn by one of the climbers.
A statement on the Greenpeace UK Twitter account said:
BREAKING: Look up London. We’re scaling the Shard, Europe's tallest skyscraper. Watch us live http://t.co/HqRjvmiHAn #DT @greenpeaceuk
— Greenpeace UK (@GreenpeaceUK) July 11, 2013
The women are understood to be planning to unveil protest artwork "that captures the beauty of the Arctic" if they manage to reach the summit, which could take them all day.
One of the climbers is Victoria Henry, 32, a Canadian living in London.
"We'll try to hang a huge art installation 310m up that will make Shell think twice before sending their rigs into the Arctic," she said.
"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."
A Shard spokesman said: "Our primary focus is on the safety of the protestors and the workers and visitors to the building. We are working with the relevant authorities to try to ensure the safety of those concerned."
Greenpeace are tweeting about the event #iceclimb
