The height of madness? Or a lofty ambition as six women scale Shard to make a point? It all ends in arrest anyway

As western Europe's tallest building, the Shard in London already gets its fair share of attention. But commuters travelling to work glimpsed an even more amazing sight: six climbers scaling the 1,017-foot (310-metre) landmark in protest against Shell's plan to drill for oil in the Arctic.

The group from Greenpeace reached the top of the building near London Bridge at around 7pm after nearly 15 hours of climbing. They unfurled a protest flag with the words "Save The Artic", before police moved in and arrested them all "on suspicion of aggravated trespass".

The climbers had begun early in the morning and initially hooked themselves on to the bottom of the building with the help of a 10-foot ladder. Once the ascent had started, police decided to monitor the climbers rather than try to intervene. A large crowd of neck-craning, eye-shielding spectators grew at the Shard's base, which is close to Shell's offices in the capital. Thousands more watched through a live stream online, which was posted with a plea to sign a petition against Arctic drilling. Greenpeace opposes Shell's proposal to drill for oil in the untouched Arctic region, which is still at the planning stage. The group pioneered "direct action" protests, which it has now enhanced using social media.

Climbers were even talked to the media as they climbed. One of the group, Victoria Henry, 32, who lives in London, said: "We are all really tired but at the same time energised because we are excited about what we are doing. We are just really proud to be raising awareness."

Greenpeace also posted a blog article written by Ms Henry before she had started climbing, which said: "Hell yeah I'm scared but I'm scaling the Shard to save the Arctic". She added that she had been "training for something that I hope will help stop one of the most heartbreaking acts of wanton environmental destruction".

The climbers had gained access to the outside of the 72-storey building at 4.20am by posing as workers for the Thameslink route being built across London. Once past security, their truck trundled straight past London Bridge station and up to the Shard, whereupon the roof flipped open and the women jumped out.

Shell said in a statement: "We respect the right of individuals and organisations to engage in a free and frank exchange of views about our operations… we only ask that they do so with their safety in mind." The company added that resources in the Arctic could be "developed responsibly" to "offset supply constraints".

After unfurling a banner at the top of the Shard, the climbers were arrested.