The fight to stop the global oil industry exploring the pristine deep waters of the Arctic has been dubbed the new cold war, and early on Friday it escalated as environmental activists from 12 countries occupied the world's second largest rig on its way from Turkey to Greenland to drill among the icebergs.

The protesters found the 52,000-tonne semi-submersible platform Leiv Eiriksson at around midnight, steaming due west at a stately six knots in the sea of Marmara, heading for the Dardanelle straits and the open Mediterranean. It took four more hours for Greenpeace to bring in its inflatables and a further 50 minutes in the choppy moonlit sea to intercept it.

Even from three miles away, the Chinese-built mobile rig, which specialises in drilling in extreme environments, looks huge. From 100ft away in the pale dawn light it is a 15-storey industrial castle, bristling with cranes, derricks, gangways, chains, spars, girders, pipes, helipads and radar. Just 10 years old, it is already rusting and its paintwork is streaked from years of drilling in harsh west African, north Atlantic and Asian waters.

The Greenpeace boats approached the vessel cautiously in the three foot swell, like fleas to the backside of an elephant. At exactly 5.31am, the 11 climbers began to leap on to its hull and headed for a ladder. The plan was to stop the vessel in its tracks not by taking over the bridge, but by radioing the captain and asking politely. Fat chance.

"This is Greenpeace, this is Greenpeace. I'm informing you that we have put climbers on your rig. I ask you stop your vessel", asked Korol Diker, a Turkish campaigner, on a VHF channel.

But the elephant barely registered. "I do not recognise you", came the captain's cutting reply and the Leiv Eiriksson steamed on.

Undaunted, the climbers made it to a gangway 80ft over the vessel's starboard stern. As four crewmen peered over the side from 30ft above them, and two more ambled over, seemingly unconcerned, the climbers made a cat's cradle of rope to hang banners and a tent from.

You can understand why the captain did not want to stop. The Scottish oil company Cairn Energy has hired the Leiv Eiriksson for around $500,000 a day and the company, run by Sir Bill Gammell, the former international rugby player, plans to spend more than $500m (£300m) in the next few months looking for oil in some of the most dangerous and coldest waters in the world. Any major delay could cost it millions and set back its plans for the Arctic by a year, because drilling is only possible in the July-October "summer window" when the ice has retreated.

Cairn, which will be the only company to drill deep wells offshore in the Arctic this year, holds 11 licences in Baffin Bay covering over 80,000 square kilometres. It plans to drill four exploratory wells to depths of around 5,000ft, the deepest ever attempted in the Arctic.

It is taking the Leiv Eiriksson and the Ocean Rig Corcovado – a drill ship now stationed near Aberdeen – as well as a fleet of backup vessels. Last year the company claimed it had discovered a "working hydrocarbon system" or possible signs of oil in Baffin Bay after drilling several 300ft wells. But it later said that the oil was not present in recoverable quantities.

Cairn Energy said in a statement: "Wherever it is active, Cairn seeks to operate in a safe and prudent manner. The Greenlandic Bureau of Minerals and Petroleum has established some of the most stringent operating regulations anywhere globally, which mirror those applied in the Norwegian North Sea. Cairn respects the rights of individuals and organisations to express their views in a safe manner."

But the Arctic venture, say environmentalists, is just the start of what is planned to be a risky offshore oil rush. Shell, Exxon Mobil, Chevron and others have licences to explore in Baffin Bay, mostly above the Arctic circle. Others, including BP and Rosneft, plan to extract oil offshore from Siberia, Alaska and elsewhere in the Arctic. The US government estimates there are 90bn barrels of oil, around 13% of the world's undiscovered reserves.

But for Greenpeace and others the risk of a devastating spill is too great, raising the spectre of the Exxon Valdez spill in Alaska in 1989 when more than 1m barrels of oil were spilt, and the $40bn disaster in the Gulf of Mexico last year. "Any Arctic spill would be very difficult, if not impossible, to contain and clean up. The company has not released a detailed spill response plan for the Arctic waters. Its latest environmental impact assessment says it has not been possible to model oil behaviour on ice. Failing to consider the impact of ice on a potential Arctic oil spill renders the EIA [environmental impact assessment] unfit for purpose," said Greenpeace campaigner Ben Ayliffe.Other leading environment groups, including the respected US thinktank Pew, plus NGOs Oceana and WWF, have all said that the oil industry is not prepared for a major pollution incident.

"This is the most controversial rig in the world because it is the only one destined to begin risky offshore drilling in the very deep waters of the Arctic this year. We have stopped it because it's blazing a trail for other major oil companies and sparking the start of a dangerous new Arctic oil rush."

Cairn says it has prepared comprehensive oil spill plans, and has put up a bond of $2bn.

Activists are now expected to dog the progress of the slow-moving Leiv Eiriksson as it passes Greece, Italy, France and Spain on its passage through the Mediterranean and into the Atlantic. It is scheduled to stop in Britain to pick up supplies before the last leg of its journey to Greenland in June.

Twelve hours after boarding the Leiv Eiriksson, the 11 activists who had occupied a gangway 80 ft above the water were forced down by a gale as the vessel entered Greek waters. No arrests were made.