The Russian Coast Guard is towing a Greenpeace ship toward the nearest port after armed officers stormed it following a protest against oil drilling in Arctic waters.
The agency said on Friday that the ship's captain refused to operate the Arctic Sunrise, so a Coast Guard ship has arrived at the scene to tow the ship to the city of Murmansk. The trip will take three to four days. A security official said activists could face terrorism charges.
One of the activists aboard the vessel, Faiza Oulahsen, said late on Thursday that about 15 armed men had boarded the Arctic Sunrise, aggressively herding 29 activists, who included six Britons, into one compartment. The vessel's captain was held separately on the bridge.
"They used violence against some of us, they were hitting people, kicking people down, pushing people," she said in a phone call from the ship.
A day earlier, two activists were arrested following an attempt to board an offshore drilling platform belonging to state natural gas company Gazprom.
The Coast Guard is part of the Federal Security Service (FSB), the main successor agency to the Soviet-era KGB.
Liliya Moroz, a representative of the FSB in Murmansk region, told Ekho Moskvy radio on Friday that the charges against the activists may include terrorism.
Under Russian law, terrorism is any action aimed at "violating general safety, frightening the public or influencing government action" that damages property or threatens the lives of others in the process. It carries a sentence of up to 10 years in prison if the person was unarmed.
Greenpeace said they had been unable to make contact with the activists since late on Thursday and had not yet received official confirmation of these charges from the security services, but said that this was the first detention of a boat and its activists in Russian waters for nearly two decades.
"This is the clear detention of people against their will," said Vladimir Chuprov, head of the energy department at Greenpeace Russia. "Terrorism is a very serious crime." The Greenpeace ship is in the Pechora Sea, an arm of the Barents Sea.