The executive director of Greenpeace International faces prison in Greenland for breaking an injunction and boarding a giant rig exploring for oil in Arctic waters.

In dramatic scenes 120km off the west coast of Greenland, a Greenpeace inflatable speedboat evaded a Danish navy warship, allowing Kumi Naidoo and two activists to clamber aboard one of the massive legs of the Leiv Eiriksson, a 52,000-tonne rig. According to Greenpeace, the crew of the rig tried to prevent them from boarding with water cannons.

The three activists are said to be 30m above sea level on a small platform. The Danish navy has launched a helicopter which arrived at the rig within the last few minutes.

In addition to the likelihood of prison, Greenpeace faces a fine of $50,000 a day after Scottish oil company Cairn Energy obtained an injunction which forbade the organisation from going within 500m of the rig. Cairn sought the injunction in Holland after 20 Greenpeace activists were arrested on the rig in the last month for stopping the rig from operating.

Before scaling the rig, Naidoo said he was calling on the rig's owner to halt drilling, and would request a copy of the rig's oil spill response plan. The document, which has not been made public, has been at the centre of a month-long campaign of direct action in the Arctic.

Naidoo said: "For me this is one of the defining environmental battles of our age, it's a fight for sanity against the madness of a mindset that sees the melting of the Arctic sea ice as a good thing. As the ice retreats the oil companies want to send the rigs in and drill for the fossil fuels that got us into this mess in the first place. We have to stop them. It goes right to the heart of the kind of world we want and the one which we want to pass onto our children."

Naidoo, 45, was a youth leader in the anti-apartheid movement in South Africa, where he was arrested several times and charged with violating provisions against mass mobilisation, civil disobedience and for violating the state of emergency. He lived underground before being forced to flee South Africa and live in exile in the UK.