Greenpeace protesters reach summit of The Shard in London
Two of them then unfurled a blue flag with "Save the Arctic" written on it.
The charity said it was protesting against plans to drill in the Arctic by the oil company Shell, although the firm said drilling there was "not new".
Six climbers accessed the Shard by climbing on to the roof of London Bridge Station. They were later arrested, the Met Police said.
They are being held on suspicion of aggravated trespass.
It took the women 16 hours to reach the top of the building after they started their ascent in the early hours.
The Shard stands at 310m (1,016ft) and has 87 storeys.
As the protesters reached the summit, Greenpeace UK executive director John Sauven said: "It is an honour to stand here at the foot of Europe's highest building and witness this remarkable achievement by these women.
"Watching them fly the flag to protect the Arctic from the top of the Shard is a remarkable sight.
"And I'm not the only one watching this today – the executives of Shell, whose offices are all around this building, simply won't be able to ignore what we have done.
"As a result of our action, 50,000 extra people have joined up to the campaign.
"If Shell continues to ignore the huge groundswell of support for protecting the Arctic then they will do irreversible damage to their reputation."
In a statement Shell said oil and gas production from the Arctic "was not new".
It said: "The Arctic region currently produces about 10% of the world's oil and 25% of its gas.
"If responsibly developed, Arctic energy resources can help offset supply constraints and maintain energy security for consumers throughout the world.
"Shell has been operating in the Arctic and sub-Arctic since the early 20th Century, giving us the technical experience and know-how to explore for and produce oil and gas responsibly."
The viewing platform in the skyscraper was closed to visitors for the safety of the public and the protest group.
A Shard spokesman said 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," he said.
The spokesman added that offices and restaurants in the complex remained open.
BBC reporter Jonathan Savage, who was at the scene, said the group climbed the edge of the building as it has a ladder-like structure, and also used ropes.
Police and ambulance teams remained at the scene.
Greenpeace said the location was chosen because it sits in the middle of Shell's three headquarters and because the building was "modelled on a shard of ice".
The climbers were live-streaming the ascent using helmet cameras.
In its statement, 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."
The Qatari-owned building, which was opened in February, contains offices, restaurants, a hotel and residential apartments.