BERLIN -- Attorneys representing the Netherlands argued Wednesday that Russia violated UN regulations when it seized the Dutch-flagged Greenpeace ship "Arctic Sunrise" and charged all on board with piracy.

The Hamburg-based International Tribunal for the Law of the Sea agreed to hear arguments on the case even though Russia refused to attend. Russia argued that even though it had ratified the 1982 UN Convention on the Law of the Sea, it stipulated it would not accept arbitration on cases it considered to be matters of national sovereignty.

Dutch attorneys asked for the immediate release of 28 Greenpeace activists, a Russian photographer and a British videographer, as well as the "Arctic Sunrise" icebreaker. All were seized by the Russian coast guard after a protest near a Gazprom-owned oil rig in the Arctic on Sept. 18.

The Dutch argued there was no evidence of piracy and that the Russians had no valid reason to board the vessel without permission, thus committing an "internationally wrongful act." All moves that followed -- like transferring the ship to Russian waters and arresting those on board -- were also "internationally wrongful," they said.

Russia's main investigative agency said two weeks ago that it had dropped the piracy charges against the jailed activists and replaced them with hooliganism charges -- meaning a maximum possible seven-year sentence instead of 15 years. At the same time Russia warned it could also file additional charges, including violence against authorities, which carries up to 10 years in prison.

As of Wednesday, however, Greenpeace said its attorneys had not yet been formally notified by Russia that the piracy charges had been dropped.

"Russian authorities propose to jail 30 men and women for two decades because a couple of peaceful protesters tried to hang a small yellow banner from the side of a 500,000-ton oil platform," Greenpeace International executive director Kumi Naidoo said in a statement after the hearing.

The Hamburg tribunal was formed after the UN Convention on the Law of the Sea came into force in 1994 to adjudicate maritime disputes. The court will first have to decide if it has jurisdiction in the case -- Russia insists it does not -- then decide on the arguments advanced by the Netherlands.

The court expects to issue its ruling on Nov. 22.