(CNN) -- Russian authorities have released from jail most of the members of a Greenpeace group arrested on their ship in September while protesting oil drilling in the Arctic.
Four were let go Friday, bringing to 18 the total number freed so far from the original 30 who were detained, the environmental group said Friday.
The so-called Arctic 30, made of 28 Greenpeace activists and two freelance journalists, were detained after two activists left their ship, the Arctic Sunrise, and attempted to scale an oil platform owned by Russian energy company Gazprom.
Workers on the Prirazlomnaya oil rig in the Barents Sea thwarted the attempt by fighting the activists with fire hoses.
Russian authorities boarded the Arctic Sunrise, drawing guns and smashing communications equipment, Greenpeace has said.
After the ship's captain refused to sail into a Russian port, authorities confiscated the vessel and towed it to the nearby city of Murmansk, where the Greenpeace group members were incarcerated.
They were transported by prison train to detention centers in St. Petersburg and charged with piracy.
The court later dropped the charge, but the 30 still face the charge of hooliganism, which carries a sentence of up to seven years in prison, according to Russian news agency RIA Novosti. The original charge of piracy carried a sentence of up to 15 years.
Greenpeace says the hooliganism charge is still "wildly disproportionate."
Russian authorities accuse the activists of trying to take over the oil platform and of endangering the lives of the company's employees. They say the activists' actions could have led to an environmental disaster.