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."
In Australia Greenpeace supporters gathered outside the Russian consulate hoping to lodge their own protest by letter.
They got no response.
(SOUNDBITE)(English) DAVID RITTER, GREENPEACE AUSTRALIA CHIEF EXECUTIVE, SAYING:
"I think we've just seen that the Russian consulate in Sydney is not prepared to accept a simple letter which raises the concerns about our activists who are being detained illegally by Russian authorities and our ship is being held.
The Arctic Sunrise was being escorted towards the port of Murmansk on Friday.
A Russian Foreign Ministry statement said the Greenpeace activists behaviour around the oil platform had the outward signs of extremist activity that could lead to deaths and other grave consequences.