Russian guards arrested the Greenpeace members in September after boarding the group's ship, the Arctic Sunrise, which said under a Dutch flag.
The activists had attempted to board a Russian oil rig operated by Gazprom subsidiary Gazprom Neft Shelf in the northern Pechora Sea.
Fourteen of the detainees were charged with piracy Wednesday and the remaining 16 were charged Thursday, RIA Novosti reported. If convicted, they each face up to15 years in prison.
"The defendants pleaded not guilty and are currently refusing to give relevant testimony," Russia's Investigative Committee said on its website.
The Greenpeace members -- from 18 countries -- were remanded for two months last week. The group Thursday appealed the decision for the defendants to be kept in pre-trial custody until Nov. 24, RIA Novosti reported.