Russian coastguards have taken 30 Greenpeace activists off their ship in the Arctic and detained them in the port of Murmansk.
Greenpeace says so far five of them have been questioned.
The activists, who were protesting against oil drilling in the Arctic, were towed for four days in their ship, the Arctic Sunrise.
Russia prosecutors have accused them of piracy after two activists climbed onto the side of an offshore oil platform.
The charge of piracy carries a prison sentence of up to 15 years in Russia, depending on the gravity of the offence, and a fine of up to 500,000 roubles (£10,000; $15,000).
The activists were initially taken to the Murmansk headquarters of Russia's Investigative Committee, modelled on the FBI.
The BBC's Daniel Sandford: "This is the first time that Greenpeace have found themselves at the criminal end of the piracy law"
On Wednesday morning, Greenpeace Russia tweeted that the activists were being detained for 48 hours and had been transferred to "different prisons in Murmansk and around".
Six Britons are among those detained. Some of their relatives told the BBC on Tuesday that they had spoken to them and they were all being well treated.
Investigative Committee spokesman Vladimir Markin said on Tuesday that "all those who assaulted the platform, regardless of nationality, will be prosecuted".
'Legitimate doubts'
The campaigners were seized on 19 September along with their ship after two Greenpeace activists tried to climb onto a Gazprom offshore platform.
The ship was raided by armed Russian men in balaclavas who abseiled down from helicopters. The ship was seized in the Pechora Sea, near the rig.The environmental organisation said in a statement on Tuesday that its protest against "dangerous Arctic oil drilling" was peaceful and in line with its "strong principles".
"Our activists did nothing to warrant the reaction we've seen from the Russian authorities," it said.
The campaigners on the ship are from 18 countries, including Australia, Brazil, Canada, Denmark, France, New Zealand, Russia, the UK and the US, Greenpeace said.
Mr Markin described the protest as "an attempt to seize a drilling platform by storm" and said it raised "legitimate doubts about their intentions". The ship "was loaded with electronics whose purpose was not clear", he said.
"It's hard to believe that the so-called activists did not know that the platform is an installation with a high hazard level, and any unauthorised actions on it can lead to an accident, which would not only endanger the people aboard it but also the ecology, which is being protected zealously," he said.
Source: BBC News
