Some 25 activists were detained by the US Coast Guard after trying to prevent a Royal Dutch Shell drilling rig from leaving its base in Seattle.
The oil giant Shell aims to resume drilling for fossil fuels in the Arctic, one of the most ecologically sensitive regions in the world.
Environmental groups say looking for oil in the remote Arctic could lead to disaster in an area that helps regulate the global climate because of the vast layers of sea ice.
Activists also say any oil spill would be nearly impossible to handle in Arctic conditions. Shell has told federal official that it would be able to remove 90 to 95 percent of any oil spill.