Google Map pinpoints Disko Island, West Greenland. Greenpeace forced a Scottish company to stop drilling off Greenland on Tuesday, when four activists occupied an oil rig.

Activists breached a 500-metre security perimeter around the Stena Don rig off western Greenland, then climbed up the rig and fastened themselves to it, police spokesman Morten Nielsen said. The breach triggered an automatic shutdown of the rig's operations.

"We caught the navy flotilla that's been following us napping, and we managed to reach the rig that's been drilling here in the Arctic," Ben Stewart, a campaigner on the Greenpeace ship, told CBC News. He said the four activists have suspended tents on the rig and are ready to camp out for days.

The activists remain on the rig and will be arrested, said police.

"When someone breaks the law — and it has happened here — the person or persons will be prosecuted," Nielsen said by telephone from Nuuk, the capital of Greenland.

The semi-submersible rig is located in the Alpha prospect in the Sigguk block, 175 kilometres off Disko Island, West Greenland and not far from Canadian waters.

Greenland is a semiautonomous Danish territory, and police have been monitoring the activists from a Danish navy ship patrolling the area.

The Danish navy said it had no immediate plans to remove the activists, saying it was up to Greenland's police to decide what to do.

"Right now we're waiting to see what happens," naval spokesman Michael Hjort told the Greenland newspaper Sermitsiaq.

"We are ready with our dinghies in case the activists fall in the water."

'Quest for media coverage'

Greenland Premier Kuupik Kleist called Greenpeace's stunt an "openly illegal act" and a "gross violation" of safety rules.

"It is really worrying that Greenpeace uses all means to break the safety rules made to protect human lives and the environment in its quest for media coverage."

Last week, the Greenpeace ship Esperanza anchored near the rig as part of a campaign to protest deepwater oil drilling.

Scotland-based Cairn Energy PLC announced at the time that it had discovered natural gas in the area but failed to find crude oil. The drilling in the Arctic has sparked condemnation from Greenpeace, whose activists are worried that an oil rush would damage the region's fragile ecosystem.

Canada ban to 2014

Three decades after one exploration effort failed to find oil, drilling in the deep ocean off Greenland's west coast resumed in 2001. Exploration had been unsuccessful until now.

Although there are more than 400 known oil and gas fields north of the Arctic Circle, many governments have been reluctant to allow drilling offshore.

Akkaluk Lynge, head of the Inuit Circumpolar Council in Greenland, told CBC News that while Inuit have concerns about drilling, Greenpeace shouldn't pretend to speak for northern people.

"[It's] unfortunate that the issue is now being centred on Greenpeace and not what Inuit themselves and the Greenlandic people themselves, want."

Lynge said Greenpeace campaigns against seal hunting demonstrate they are not friends of Inuit.

Canada, meantime, has banned new deepwater drilling in the Arctic until 2014 at the earliest, and the National Energy Board is reviewing the standards under which any licences would be granted.

There is a Canadian connection to Cairn's discovery. In addition to the find being only a few kilometres from Canadian waters, as of March the Canadian Pension Plan Investment Board owned 158,000 shares in Cairn and nearly half a million shares in its sister company, Cairn India.