Armed Russian officials boarded the Greenpeace ship Arctic Sunrise on Thursday.
The vessel, seen here in file footage, was circling an Arctic oil platform at the time.
They're protesting against environmental risks from increased energy exploitation in the region.
Moscow accused the activists of aggressive and provocative behaviour after two of them scaled the side of the platform.
At least 25 activists on board the have been arrested.
The head of the Greenpeace Russia Arctic programme says he's puzzled by Moscow's actions.
(SOUNDBITE)(Russian) VLADIMIR CHUPROV, CHIEF OF GREENPEACE RUSSIA ARCTIC PROGRAMME, SAYING:
"These actions by the Russian authorities are very strange and the main question is who is going to benefit from it? For the moment it has a bad influence on the country's political image, on the Russian president's political image. From our point of view that doesn't do him any good, even from a political point of view."
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