Greenpeace on Thursday urged Russia to release its crew members after investigators reduced the charge against them from piracy to hooliganism over their protest on an Arctic oil platform.

"Our general position has not changed: the investigation must wind up this laughable case, apologise and set them all free," Greenpeace lawyer Anton Beneslavsky told AFP.

Russia's powerful Investigative Committee, which probes serious crimes, late Wednesday announced it was softening the charge against the 30 crew members.

The maximum punishment for piracy in an organised group is 15 years while hooliganism can carry a sentence of seven years.

The piracy charges against the crew members from 18 different countries on a Dutch-flagged ship prompted international protests and pushed the Netherlands to launch legal proceedings against Russia.

Russia said Wednesday it would boycott maritime court hearings sought by the Netherlands seeking the release of the crew and the ship.

Greenpeace lawyer Beneslavsky said the hearings before the International Tribunal for the Law of the Sea would be unaffected by the change of charge.

"The basis for proceedings at the international tribunal is the seizure of the ship by Russia. How the side that seized the ship justifies its actions does not influence the jurisdiction of this case."

The new charge of hooliganism is the same used against the Pussy Riot punks for their protest performance against President Vladimir Putin in a Moscow church which landed two band members in prison for two years.

"The Greenpeace action in the Arctic has been equated with the Pussy Riot in the Church of Christ the Saviour," rights lawyer Pavel Chikov wrote on Twitter.

The crew members, including two journalists, have been detained behind bars in the northern Murmansk region after two activists scaled a state-owned oil platform to protest against Russia's energy prospecting in the Arctic, one of Russia's top economic projects.

The government is preparing to host the Winter Olympic Games in Sochi in February, and many see the tough line against Greenpeace as a warning to any activists thinking about protesting during the games, the Kremlin's key prestige project.

Last month, Putin fiercely pledged to protect Russia from foreign influences, saying its sovereignty and independence were "red lines" that could not be crossed.

Maria Lipman, a political analyst with the Carnegie Moscow Center, outlined what she sees as the Kremlin's line of thinking: "People who dare to tell Russia what it should or should not do should be punished."

Yet Greenpeace and political commentators said investigators moved to cancel a clearly excessive charge after mounting international pressure.

Even Putin said publicly that the activists were "obviously not pirates."

"They finally realised that the idiocy of the Investigative Committee is disgracing us all," opposition leader and lawyer Alexei Navalny wrote on Twitter.

The crew members are currently being held under pre-trial detention until November 24.

Some predicted they could now avoid a prison term or even be released ahead of the trial.

"It's possible they will get suspended sentences," said Lipman.

Navalny said he thought the international activists could receive a minor punishment.

"They'll take away the foreigners' passports, put them under house arrest and then hand them community service," he wrote on Twitter.

Navalny, a top Putin critic, this month had his five-year prison sentence for embezzlement changed to a suspended term that bars him from political office in the foreseeable future.

But Greenpeace cautioned that the activists are not experiencing any immediate benefits.

"The charge is changed but that does not make the team's treatment any lighter. They are still under a grave charge," said Alexei Kiselyov, a Greenpeace representative.