A group of activists from Greenpeace on Thursday scaled the Shard, the tapered 310m glass tower next to London Bridge station, as a protest against oil and gas drilling in the Arctic.

In an action that was perhaps inevitable at some point after the completion last year of western Europe's tallest building, visible across much of central London, the six climbers began their ascent in the early hours of the morning. The activists reached the top of the building at around 7.10pm.

The Twitter feed for Greenpeace UK carried a photograph of the activists, all women, in their climbing gear before they began the protest, naming them as Wiola Smul (23) from Poland, Ali Garrigan (27) from the UK, Sabine Huyghe (33) from Belgium, Sandra Lamborn (29) from Sweden, Victoria Henry (32) from Canada and Liesbeth Deddens (31) from the Netherlands. Police said all six had been arrested on suspicion of aggravated trespass.

The organisaton also carried a blog post from one of the climbers, Henry, explaining her motivation: "I'm training [the post was written before the climb] for something that I hope will help stop one of the most heartbreaking acts of wanton environmental destruction – drilling for oil in the Arctic. I'm over the moon that I can play any part in the mass resistance to this horrifying practice," she wrote.

Another tweet read: "Look up London. We're attempting to scale the Shard, Europe's tallest skyscraper."

A picture with that tweet appeared to show the group using ladders to gain access to the bottom of the 72-storey office and residential block from the roof of London Bridge station. A live webcam was following their progress up the tower.

In a later statement, Greenpeace said the Shard was chosen because it was in sight of the three London bases of the energy giant Shell, which is involved in offshore drilling in the Arctic. When the climbers reached the top of the building they aimed to "hang a huge work of art that captures the beauty of the Arctic", it added.

The statement continued: "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."

Shell released a statement that said it respected the right of Greenpeace to engage in an "exchange of views" about their operations. It said oil and gas production in the Arctic was not new.

"If responsibly developed, Arctic energy resources can help offset supply constraints and maintain energy security for consumers throughout the world," said Shell. "We work extensively with global Arctic stakeholders to research and develop standards and best practice on biodiversity, ecology, marine sound, oil spill prevention and response, safety and health."

A Metropolitan police spokesman said: "We were called at 4.20am today to a group of protesters attempting to climb up the Shard. We are in attendance and monitoring the situation along with British transport police."

A spokesman for the building, designed by the Italian architect Renzo Piano, said: "The Shard is being used by protesters as part of a campaign. Our primary focus is on the safety of the protesters 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 said the climbers used a combination of traditional mountain climbing techniques, rope access techniques used by commercial building climbers and some free climbing. The building has an external metal frame providing handholds and crossbeams all the way to the top.

Each stage of the climb would require the lead climber to free-climb a section of the building. Once they reached a secure position, they would fix a rope to the external skeleton of the building and the rest of the group would follow using the rope. All the climbers were wearing harnesses, meaning they would not fall more than six metres if they slipped.

The British Mountaineering Council training officer Jon Garside said there was a "rich history of people climbing buildings both for recreation and for protests". He said the climbers appeared to have standard kit used by workers on tall buildings around the world and that what they were doing, while inherently risky, was fairly common practice. "Any Londoner will see all the time access workers abseiling down large buildings like the Shard to clean the windows," he said.