The viewing platform of the Shard has been closed to visitors as Greenpeace activists scale Britain's tallest building in protest at drilling in the Arctic.
Six women, who evaded security guards at the 72-storey Shard in central London early this morning, said their climb was intended to put Shell and other oil companies in the spotlight.
A Shard spokesman said that in the interests of public and protester safety, the building's emergency response team had advised that "The View", the platform on floors 68, 69 and 72 which gives a view of London from 800ft (244m) up, should be closed with immediate effect.
"We apologise to guests for the inconvenience caused and The View will be pleased to honour their tickets either later on today or on a different date."
The spokesman added that all three restaurants and offices in the building remained open.
Greenpeace said the protesters were "artists and activists", adding: "If the six women reach the top - 310m above the pavement (1,017ft) - they will attempt to hang a huge work of art that captures the beauty of the Arctic.
"They chose to climb the Shard because it towers over Shell's three London offices, including the oil giant's global headquarters on the South Bank of the Thames.
"Shell is leading the oil companies' drive into the Arctic, investing billions in its Alaskan and Russian drilling programmes.
"A worldwide movement of millions has sprung up to stop them, but Shell is refusing to abandon its plans."
Metropolitan Police are at the site, a spokesman said.
The demonstrators are live-streaming the climb from helmet cameras, with birds-eye views of their ascent being broadcast live at www.iceclimb.savethearctic.org.
They said if they do hang the Arctic artwork it will be the highest successful installation of an art project since Philippe Petit tightrope-walked between the twin towers of the World Trade Center in 1974.
One climber, Victoria Henry, 32, a Canadian living in Hackney, London, said this morning: "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."
Asked about the Shard protest during his call-in show on LBC 97.3 radio, Deputy Prime Minister Nick Clegg said: "Of course what they are doing looks extraordinary on the telly, (but) I don't actually support them doing something like that, which is going to tie up a lot of police time and is obviously quite dangerous.
"I really do think that they could get their point across to the companies which they are seeking to address in a different way."
Along with Ms Henry, the other five women climbing the Shard are Ali Garrigan, 27, from Nottinghamshire, who lives in Manchester; Wiola Smul, 23, from Poznan, Poland; Sabine Huyghe, 33, from Ghent, Belgium; Sandra Lamborn, 29, from Stockholm, Sweden; and Liesbeth Deddens, 31, from Groningen, Netherlands.