Around 30 Greenpeace activists have been accused by Russian prosecutors of piracy and will be prosecuted for trying to board an Arctic oil platform.
Russia’s Investigative Committee, modeled on the FBI, will question the activists.
The Greenpeace ship, Arctic Sunrise, is being towed to the port of Murmansk.
Committee spokesman Vladimir Markin said “all those who assaulted the platform, regardless of nationality, will be prosecuted”.
The campaigners were detained on Thursday along with their ship after two Greenpeace activists tried to climb onto a Gazprom offshore platform, in a protest against oil drilling in the Arctic.
Arctic Sunrise was raided by armed Russian men in balaclavas who abseiled down from helicopters. The ship was seized in the Pechora Sea, near the rig.
Greenpeace said the Russian authorities “are holding 30 of our activists”, though that figure was not confirmed by Russian officials.