STOCKHOLM -- Swedish police on Thursday arrested six Greenpeace activists after they boarded an icebreaker off Sweden's southeast coast, forcing authorities to tow it back to land.

The Greenpeace action was the second of its kind this week and was staged to protest oil and gas exploration in fragile Arctic waters.

Police in Blekinge said the activists are accused of trespassing after having approached the Shell Oil Co.-contracted Nordica in rubber boats and climbing onto it. Some of them also chained themselves to the vessel.

"The arrests were made in a calm manner and the environmental activists were then transported in to land," police said in a statement.

Greenpeace is calling on Shell to abandon its plans to open up the waters off the Alaska coast for oil drilling, insisting an oil spill there would be detrimental for the environment and impossible to clean up because of the extreme weather conditions in the region.

On Tuesday, the group staged a similar action as the ship was due to depart from Finland, with Greenpeace saying it managed to delay it by up to 10 hours.

Two Finnish icebreakers, the Fennica and Nordica, are contracted to support oil and gas production operations on the northern Alaska coast from 2012 to 2014.