The 30 activists from 18 countries were on a Greenpeace ship, the Arctic Sunrise, which was seized last week by the Russian Coast Guard. The ship was towed Tuesday into a small bay near Russia's Arctic port of Murmansk and the activists were bused to the local headquarters of Russia's Investigative Committee late at night for several hours of questioning and then into a detention facility.
The Investigative Committee, Russia's main federal investigative agency, said its agents will question all those who took part in the protest and detain the "most active" of them on piracy charges. Piracy carries a potential prison sentence of up to 15 years and a fine of 500,000 rubles (about $15,500).
Two activists tried to climb onto the Prirazlomnaya platform on Thursday and others assisted from small inflatable boats. The Greenpeace protest was aimed at calling attention to the environmental risks of drilling for oil in Arctic waters.
"When a foreign vessel full of electronic technical equipment of unknown purpose and a group of people calling themselves members of an environmental rights organization try nothing less than to take a drilling platform by storm, logical doubts arise about their intentions," Investigative Committee spokesman Vladimir Markin said in a statement.
He said the activists posed a danger to operations on the oil platform. "Such activities not only infringe on the sovereignty of a state, but might pose a threat to the environmental security of the whole region," Markin said.
The oil platform, the first offshore rig in the Arctic, was deployed to the vast Prirazlomnoye oil field in the Pechora Sea in 2011, but its launch has been delayed by technological challenges. Gazprom has said it was to start pumping oil this year, but no precise date has been set.
Greenpeace insisted that under international law Russia had no right to board its ship and has no grounds to charge its activists with piracy.
"Peaceful activism is crucial when governments around the world have failed to respond to dire scientific warnings about the consequences of climate change in the Arctic and elsewhere," Greenpeace International executive director Kumi Naidoo said in a statement.
"We will not be intimidated or silenced by these absurd accusations and demand the immediate release of our activists," he added.
One Greenpeace activist told The Associated Press that Coast Guard officers hit and kicked some activists when they stormed the Greenpeace vessel.
The Arctic Sunrise was anchored Tuesday in Kulonga Bay near Severomorsk, the home port of Russia's Northern Fleet, 25 kilometres (15 miles) north of Murmansk.
Greenpeace, which had limited contact with those on board, said they were all fine and that some made calls to friends and family.
Diplomats were allowed to board the Arctic Sunrise for two hours to meet with activists from their countries. Late Tuesday, the activists were driven to the Investigative Committee's headquarters in Murmansk for several hours of questioning, and after 2 a.m. local time Wednesday (2200 GMT Tuesday) Associated Press reporters saw them bused to a detention facility.
Officials wouldn't comment on the course of the investigation.
Greenpeace said the activists hailed from Argentina, Australia, Brazil, Britain, Canada, Denmark, Finland, France, Italy, the Netherlands, New Zealand, Poland, Russia, Sweden, Switzerland, Turkey, Ukraine and the United States.
Berry and Vladimir Isachenkov reported from Moscow.