Despite its expertise at risky protests, Greenpeace's head admitted he was "extremely surprised" by Russia's response in the Arctic and never imagined activists would face piracy charges and years in jail.

"We were extremely surprised," International Executive Director Kumi Naidoo said in an AFP interview from the group's global headquarters in Amsterdam.

"Last year we did exactly the same action, at the same oil rig, and they did nothing," Naidoo said.

The 30 people aboard Greenpeace's Arctic Sunrise from 18 countries have been charged with piracy after trying to scale the Gazprom rig and are being held in pre-trial detention for at least two months.

Underlining the unusually harsh response to the non-violent protest, the piracy charge carries a possible 15-year jail term with Russia saying more charges could follow.

"I have to say quite honestly we never anticipated piracy being a charge," the South African former anti-Apartheid activist said during a telephone interview.

Greenpeace has called the prosecutions "the biggest threat to peaceful protest since the sinking of the Rainbow Warrior", the environmental group's ship that was blown up by French secret service agents in New Zealand in 1985, killing a photographer.

During the 2012 Arctic protest, which Naidoo himself took part in, Russian coastguards "were being berated by Gazprom security to intervene and arrest us and they completely refused."

Naidoo on Wednesday offered himself as security to the Russian authorities so that the activists can be freed on bail.

He said he hopes to have a response to his offer in three days, and has proposed for Russia also to press piracy charges against him for last year's protest.

"When we did the action last year we looked at all the possible responses, but I have to say piracy was just not something we've ever looked at," he said.

"You have to be armed, violent, seeking personal gain."

Greenpeace actions can be expensive and risky because they frequently aim to highlight what they feel is an environmental problem in a remote part of the world.

So each action is planned down to the last detail, "cost-benefit analysis and so on", said Naidoo.

"We look at different risks, legal risks, safety risks, for property, for people and so on and we look at financial risk as well," including now-mounting legal costs, said Naidoo.

Greenpeace critics accuse it of staging headline-grabbing protests that change little, while ideological enemies accuse it of being opposed to progress.

"Yes, we are being attacked, but I learnt as a 15-year-old activist in apartheid South Africa that the struggle for justice is not a popularity contest," he said.

But, Naidoo points out, "peaceful non-violent direct action like what's happening in Russia now is not even 20 percent of what we do."

These days it is just as important to engage positively with governments and big business.

"Good activism is having a lot of different tactics and strategies in your tool box," he said.

Grassroots activists in turn criticise Naidoo for going to such events as the World Economic Forum's annual meeting of business and political leaders in Davos, Switzerland.

Naidoo says that the dual strategy of lobbying and protesting is summed up in one of Greenpeace's mottos: "No permanent friends, no permanent enemies."

"If Coca-Cola or Unilever says we're going to phase out HFC (greenhouse) gases from our mass refrigeration, then we thank them."

"Some of our colleagues they say you're legitimising big corporate entities, they're part of the problem."

"But if a company does the right thing, if a government does the right thing, we'll say let's support them, let's encourage them, even if it's not 100 perfect."

Nevertheless, Greenpeace remains committed to its roots in the non-violent Quaker movement, and its notion of "bearing witness".

"The notion is that those of us who have the ability to witness, document and share with the world an injustice, or in our case an environmental injustice or crime taking place, have an obligation to try do that," Naidoo said.

"Then people are left with the moral choice of whether they want to stand up against the injustice or observe and be indifferent to it."