The sight of six tiny figures inching up the smooth glass exterior of Europe's tallest building was almost too much for some of the onlookers gathered below.

"Rather them than me," said Nigel Steward, 55, who was on a day trip from home in Surrey with his wife and her sister. "We were on the viewing platform earlier and could see them coming up. It was quite a sight."

Steward was one of hundreds of people craning their necks at the base of the Shard in central London as a team of six environmental campaigners scaled the 310-metre glass tower above London Bridge to draw attention to Shell's oil and gas drilling plans in the Arctic.

Speaking on a mobile phone, perched two thirds of the way up the building, one of the climbers, Victoria Henry, 32, told the Guardian the climb had been exhilarating but challenging.

"We are all exhausted because we are carrying a lot of weight and have been going since early this morning but it is going really well.

"It seems that our message about what is happening in the Arctic is really getting out there, which is great."

The idea for the protest was hatched two months ago when several Greenpeace activists visited the Shard's viewing platform. "We were standing there and realised that we could see three Shell buildings including its HQ," said Ben Stewart, head of media at Greenpeace. "We realised if we can see them, they can see the Shard, so it seemed like the perfect place to get our message across to them."

A group of female Greenpeace activists took up the challenge under the code-name Sigmund – "after Freud and his theories on why people climb tall buildings" – and a detailed plan was put in place.

Stewart said: "It has taken a lot of work and the people involved have not had much sleep."The activists – all experienced climbers – arrived at the base of the Shard at 4.20am yesterday in a specially designed van with a "flip roof". This allowed them to lock themselves inside, push a ladder through a hole in the roof and clamber onto the top of London Bridge station's ticket hall. From there they could access the bottom of the Shard and start the climb.

Greenpeace said the women – who were sustained by a diet of energy bars and cheese bagels – used a combination of traditional mountain-climbing and rope access techniques used by commercial building climbers. Each stage of the climb required the lead climber to free-climb a section of the building. Once she reached a secure position, she fixed 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.

Five stopped 18 metres below the top before the youngest of the group – Wiola Smul, a 23-year-old student from Poland – made the final ascent. She reached the top before 7pm but as the climb had taken several hours longer than the women had expected the group did not have time to unveil the giant polar bear down the side of the building, although they did unfurl a "Save the Arctic" banner, to cheers from supporters below. The women then made their way inside the building.

Scotland Yard said that all six protesters had been arrested on suspicion of aggravated trespass.

Greenpeace activists unveil a flag at the top of the Shard as part of a campaign to draw attention to Shell's Arctic drilling plans. Photograph: Anthony Devlin/PA

Apart from the climb itself Greenpeace set up a sophisticated media operation with a live stream from cameras mounted on the women's helmets and a live radio commentary over it. Organisers say they had expected 200 to 300 people to log on at any one time but as the climbers were reaching the top of the building there were more than 13,000 following the event in real time."The media interest has been fantastic and I think it has really helped to raise the issue of what is happening in the Arctic," said Stewart.During the afternoon, as the climbers continued their ascent Greenpeace issued a statement outlining its concerns about Shell's plans for drilling in the Arctic."Shell is leading the oil companies' drive into the Arctic, investing billions in its Alaskan and Russian drilling programmes," it read. "A worldwide movement of millions has sprung up to stop them, but Shell is refusing to abandon its plans."

Shell said it respected the right of Greenpeace to engage in an "exchange of views" about their operations, but added: "If responsibly developed, Arctic energy resources can help offset supply constraints and maintain energy security for consumers throughout the world." "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 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."

Below, as the women neared the top, there was a mixture of bemusement and admiration among commuters heading into London Bridge station. "They must be bloody crazy going up there," said Andrew Symonds, 26. "It gives me the shakes just watching."

But scaffolder Johnny Arthur, 59, had nothing but admiration. "I say good luck to them. They want to make a statement and they are prepared to do something about it – I think it is great because people have to realise there are consequences to what we are doing. I don't want to be in the position in a couple of years where we are showing children pictures of tigers or polar bears and saying 'we used to have animals like this'."