Armed Russian Coast Guard officers have boarded a Greenpeace ship after Moscow accused the environmentalist group of "aggressive and provocative" behaviour.
The campaign group said 25 activists were arrested following a protest against Arctic oil drilling operations.
Greenpeace said coast guard personnel rappelled onto the ship from a helicopter.
"Greenpeace International activists locked inside the radio room said they saw other activists detained on their knees with guns pointed at them," it said in a statement.
A tweet that Greenpeace said was posted from on board the ship before communications were cut, read: "This is pretty terrifying. Loud banging. Screaming in Russian. They're still trying to kick in the door."
Two Greenpeace activists scaled the side of the Gazprom-owned Prirazlomnaya platform on Wednesday and were arrested.
Moscow summoned the Dutch ambassador to discuss the issue. Greenpeace is based in the Netherlands and its Arctic Sunrise, which had 25 people aboard, is a Dutch-registered vessel.
The protest is against the environmental risks posed by increased energy exploitation in the Arctic, estimated to hold 13% of the world's undiscovered oil reserves and 30% of its undiscovered natural gas.
Prirazlomnaya is Russia's first Arctic offshore oil rig and a sensitive project in a country that has made tapping the region's resources a priority. Greenpeace activists last boarded the platform in August.
"The intruders' actions were of aggressive and provocative character and had the outward signs of extremist activity that can lead to people's death and other grave consequences," the Russian foreign ministry said in a statement issued before news of the armed raid on the boat.
Greenpeace International executive director Kumi Naidoosaid: "We ask President Putin to restrain the Coast Guard and order them to holster their guns and withdraw.
"We are a peaceful organisation and our protest has done nothing to warrant this level of aggression."
Global majors including ExxonMobil, Eni and Statoil plan to work in Russia's Arctic waters and Norway is pushing ahead with its own drilling plans.
But technical difficulties, high costs and mishaps as well as environmental campaigns have caused hesitation among some players.