Russia has put 30 activists, detained while protesting Arctic oil exploration, into pre-trial centers in the Murmansk area, according to the environmental group. Russia says the "most active" could be charged for piracy.
Greenpeace described Russia's detention of 30 activists from 18 countries in the Murmansk area as "completely incomprehensible" on Wednesday. Russian officials said prosecutions could follow regardless of citizenship.
Piracy under Russian law can result in up to 15 years jail.
Russia seized the group and its ice breaking protest ship in part of the Arctic known as the Pechora Sea last Thursday as the activists tried to board an oil platform deployed in recent years by the Russian oil concern Gazprom.
Greenpeace said some of its 30 activists and crew had been questioned until the early hours of Wednesday. They were then taken to various detention centers in and around Murmansk, said spokesman Aaron Gray-Block.
" No formal charges have been laid yet," he said.
"Diplomats had gathered outside with media and police, while some 15-16 Investigators had arrived earlier, together with translators," he added.
Russia begins 'criminal' probe
The chief spokesman for Russia's equivalent to the American FBI, Vladimir Markin of Russia's powerful Investigative Committee, said on Wednesday that authorities in Murmansk had launched a criminal probe for piracy "undertaken by an organized group."
Markin argued that the Greenpeace ship, Arctic Sunrise, was in the "exclusive economic zone of the Russian Federation" when it was boarded by agents of Russia's Federal Security Service (FSB) last Thursday.
A Russian tug towed the ship and its detained crew into Mumarsk harbor on Tuesday.
Unlawful, says Greenpeace
Greenpeace's international executive director Kumi Naidoo in an emailed statement said: "Any charge of piracy against peaceful activists has no merit in international law."
Greenpeace Russia's spokesman Ivan Blokov told the Russian news agency Interfax that the continuing detention of the activists was "completely incomprehensible."
In last week's protest, two activists tried to scale the platform but eventually slipped into the freezing water and were recovered by the Russian coastguard.
FSB agents then seized the Arctic Sunrise.
Rising average temperatures in the Arctic attributed to climate change and opening up of sea lanes have prompted energy producers to seek access to oil and gas reserves.
Greenpeace argues that oil spills and lack of precautions pose severe dangers to icy regions that are home to polar bears, walruses and rare seabirds.
ipj/rg (AFP, dpa)