Russian authorities have eased criminal charges against the crew of a Greenpeace ship, changing an indictment for piracy to a lesser charge of hooliganism.

The Russian government's Investigative Committee announced the new charge on Wednesday. Under the previous piracy indictment, the crew of 28 Greenpeace activists and two freelance journalists could have faced 15 years in prison for staging a protest at a Russian oil rig on September 18.

The hooliganism charge carries a maximum sentence of seven years. Greenpeace spokesman Vladimir Chuprov rejected the move, saying hooliganism is "still a wildly disproportionate charge" that the environmental group will contest "as strongly as we contested the piracy allegations."

Russian police have detained the 30 crew members in the northern region of Murmansk since the incident off the Russian coast. The activists attempted to scale the Russian state-owned oil rig to protest drilling in the Arctic.

The Russian investigative agency dismissed Greenpeace's claim that it was acting peacefully, saying it is a crime to try to seize an oil rig. It also warned that some crew members could face an additional charge of using violence against Russian authorities.

Russian President Vladimir Putin said last month he did not think the Greenpeace protest was an act of piracy, but he added that he believes it violated international law.