The 30 Greenpeace activists currently in jail in Russia were arrested illegally, Dutch lawyer René Lefeber told the International Tribunal for the Law of the Sea on Wednesday.

The activists were aboard the Arctic Sunrise when it was boarded by Russian coastguard officials in late September while campaigning against oil drilling. Two activists had climbed aboard a Gasprom rig drilling in the Pechora Sea, northwest of Russia’s Arctic coastline.

Two of the activists are Dutch nationals and the ship sails under the Dutch flag.

Originally, the Arctic 30 were charged with piracy but this was changed to hooliganism in late October. The new charge carries a maximum sentence of seven years.

The Dutch began their arbitration process at the Tribunal around the same time.

On Wednesday, Lefeber told the Tribunal in Hamburg that boarding a ship in an exclusive economic zone is not allowed, except where explicit conditions are met, news agency ANP reports.

According to Lefeber, these conditions were not met when the Arctic Sunrise was boarded, making the action illegal. This makes every following action illegal, including the arrest of the ship’s crew.

The Tribunal’s judgement is expected on November 22.