The Greenpeace activists suspended themselves from tents on ropes off the rig for more than 40 hours. They were arrested early Thursday morning and taken into police custody in Greenland. (Greenpeace)

Four Greenpeace activists have been arrested after they climbed onto an oil rig off Greenland this week, trying to halt Arctic offshore drilling there.

The four activists were arrested early Thursday and are currently being held in police custody in Greenland, according the environmental activist group.

The protesters had breached a 500-metre security perimeter around Cairn Energy PLC's Stena Don rig off the western Greenland coast on Tuesday, climbed onto the rig and fastened themselves onto it with hanging tents suspended from ropes.

The security breach triggered an automatic shutdown of the rig's operations.

Early Thursday morning, however, after being on the rig for more than 40 hours, the protesters had to leave the rig when stormy conditions made the occupation dangerous, said Ben Stewart, another Greenpeace campaigner who was in a nearby ship.

"We made the call that we would bring our activists down. They've managed to stop all drilling in this area for two days," Stewart told CBC News on Thursday.

"Once we decided to bring them down, we asked police if we could take our boats in to bring them down. Unfortunately, the police refused that, even though it would be the safest option. So [the activists] had to go up instead and climb onto the … platform. And there they were arrested and they've been taken to the mainland."

Greenpeace officials say they believe the arrested protesters are fine, but do not know when they will be released from custody.

Ship anchored near rig

Stewart said the ship he was on has been shadowed by two Danish naval ships, adding that he is unsure if he and others will be arrested as well.

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.

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

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 expanded drilling would damage the region's fragile ecosystem.

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.

With files from The Associated Press