Russia said Wednesday it had dropped the charge of piracy against 30 Greenpeace activists detained while protesting against Arctic drilling, accusing them instead of "hooliganism".

The Investigative Committee, the Russian agency in charge of probing serious crimes, said it had reclassified the crime as hooliganism, which carries a lesser sentence.

The crew of Greenpeace's Arctic Sunrise ship, who come from 18 countries including Britain, Australia, Spain and Russia, had been facing up to 15 years in a penal colony if found guilty of piracy.

The hooliganism charge -- which was also used against the Pussy Riot punks -- carries a maximum sentence of seven years in a penal colony.

Greenpeace on Wednesday said the charge was still "wildly disproportionate" and called for its activists to be immediately released.

Russia charged the crew of the Dutch-flagged ship with piracy early this month after they attempted to scale a state-owned oil platform in a protest against drilling in the Arctic.

They are being held in pre-trial detention until November 24 in the northern Russian region of Murmansk.

The case has sparked an international outcry with stars such as British actor Jude Law joining vigils outside Russian embassies.

The Netherlands has taken Russia to the world's maritime court in a bid to free the crew members, but Moscow said earlier Wednesday it would boycott the hearings before the International Tribunal for the Law of the Sea (ITLOS).

The Russian announcement marked another step in an escalating feud between the economic partners that has also witnessed the brief detention of a Russian diplomat in The Hague and Moscow's threat to ban some Dutch imports.

Greenpeace's Russian office said in a statement sent to AFP the activists "should be released immediately."

"The Arctic 30 are no more hooligans than they were pirates. This is still a wildly disproportionate charge that carries up to seven years in jail," said Greenpeace's Vladimir Chuprov.

"The Arctic 30 protested peacefully against Gazprom's dangerous oil drilling and should be free."

Putin has said the activists "violated the norms of international law," by climbing up the oil platform in the Barents Sea, owned by energy giant Gazprom.

During the protest, two activists climbed up the platform using ropes while workers hosed them with cold water.

Russian border guards then stormed the Greenpeace ship and locked up the crew members before towing them to the port of Murmansk, nearly 2,000 kilometres (1,250 miles) north of Moscow and above the Arctic Circle.

In its statement, the Investigative Committee said that whatever the activists' motive, "it is obvious that in this specific situation we cannot talk of 'peaceful actions.'"

The Committee said the crew were charged with acting in a group and with "using objects as weapons".

It said it was still examining possible scenarios including "seizing the platform for financial motives and terrorist motives and carrying out illegal scientific activity and espionage."

It added that it had not ruled out additional charges for "serious crimes, specifically the use of force against an official."

It was not clear what use of force it was referring to, but the investigators earlier this month accused the activists of ramming coastguards' boats.

The Investigative Committee's Wednesday statement did not repeat a previous allegation that it had found "narcotic substances" on the ship, something Greenpeace denied.

The Pussy Riot punks were charged with hooliganism for staging a peaceful protest against Putin in a Moscow church.

Two of the all-female rock band's members are serving two years in a penal colony. A third was freed on appeal with a suspended sentence.