Six Greenpeace protesters set out to scale the tallest building in western Europe, The Shard, on Thursday.
Greenpeace said it was protesting against Shell's Arctic drilling.
Billed as the #Iceclimb, Facebook users and followers of other social media were invited to follow their progress live here.
The message posted on the site and signed "Victo, Ali, Sabine, Sandra, Liesbeth & Wiola," explained the choice of locations thus:
"This building — modeled 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. Join in by sharing this far and wide."
The BBC cited the Met Police as saying the group accessed 1,016-foot skyscraper in central London by climbing on to the roof of nearby London Bridge Station.
The oil company, whose global headquarters on the south bank of the Thames, responded to the protest by saying that Arctic drilling was "not new."
The Daily Mail cited Greenpeace as saying the protesters were "artists and activists" who would attempt to hang an artwork that "captures the beauty of the Arctic" from the top of The Shard.
Greenpeace told the paper:
"Shell is leading the oil companies' drive into the Arctic, investing billions in its Alaskan and Russian drilling programs. A worldwide movement of millions has sprung up to stop them, but Shell is refusing to abandon its plans."
Among those who tweeted their support for the climb early on were Radiohead's Thom York, a noted environmental activist, and Annie Lennox.