By Thomas Escritt AMSTERDAM, Oct 4 (Reuters) - The Netherlands launched legal proceedings against Russia on Friday, saying it had unlawfully detained Greenpeace activists on a Dutch-registered ship for protesting against oil drilling in the Arctic. Two Dutch citizens were among 30 people on board the Arctic Sunrise, a Greenpeace ship, which was seized by Russian authorities last month after activists staged the protest at the Prirazlomnaya offshore oil platform. Russian authorities have pressed piracy charges, which could result in prison sentences of 15 years, against the activists. The Russian government declined immediate comment. "The Netherlands today began an arbitration procedure on the basis of the (United Nations) Convention on the Law of the Sea," Foreign Minister Frans Timmermans wrote in a letter to the Dutch parliament. Greenpeace said the activists had been engaged in a peaceful protest in international waters to highlight the environmental risks posed by drilling in Arctic waters. "Russian officials will now be called to explain their actions before an international court of law, where (they) will be unable to justify these absurd piracy allegations," said Greenpeace lawyer Jasper Teulings. The Dutch government contested the "unlawful manner" in which the ship was intercepted, Timmermans said, and would seek the release of all its passengers, who include 28 activists and two freelance journalists. Apart from the Dutch citizens, the group includes one American, one Argentinian, one Australian, two Britons, two Canadians, one Dane, one Frenchman, one Italian, two New Zealanders, two Russians, one Swiss, and one Turk, according to Greenpeace The Dutch government could "ask the Tribunal on the Law of the Sea for temporary measures for the release of the ship and its passengers," if the arbitration does not result in their release, he said. The Hamburg-based International Tribunal for the Law of the Sea was set up in 1996 to settle maritime legal disputes between states. (Reporting By Thomas Escritt, additional reporting by Steve Gutterman.