'A joke, but a very unfunny one," is how Phil Ball, one of the Arctic 30, described hearing he faced a charge of piracy. "Even when lawyers, cellmates and yes, even the prosecutor, made jokes about parrots and tried to say 'Arrr' in a Russian accent," the father of three young children wrote, in the latest letter to be received from the bleak Murmansk jail cells which are holding the Greenpeace Arctic 30 in their eighth week of imprisonment.

In it Ball, 42, a cameraman from Oxford, said he was struggling to cope with the new charge the 28 activists and two journalists have now been told they face, an accusation of hooliganism that carries a sentence of seven years. Ball said he is appalled at the wording of the charge, which defines hooliganism as an act in "contempt of society", when he fervently believed the Greenpeace protest he was involved was quite the opposite. He accused "greedy mega-rich oil companies" of being themselves the hooligans, ignoring runaway climate change and acting "in contempt of the societies of our children and grandchildren".

Ball's family will have been heartened by the defiant tone of his letter, although the drawings he has made during his incarceration reveal the depressing conditions in which he and the others are being held. And he also talks movingly of his fears about the prospect of a conviction that would see him facing years in jail and missing the childhood of his "three cheeky monkeys".

"My sons, aged seven and nine, will be teenagers, and my little girl will have forgotten who I am if I get out of here in seven years. Pretty unfunny, too," he wrote.

It's clear from the few notes and messages coming out from Russia that Ball's worries are shared by all of the Arctic 30, in the face of the Russian authorities' continuing defiance of growing international pressure to release the men and women from 18 different countries who were on board the Greenpeace Arctic Sunrise on 18 September, when they were arrested at gunpoint.

They have received political support from all over the world – from Hillary Clinton, Angela Merkel and Archbishop Desmond Tutu, to the vice-president of Iran, Eshaq Jahangiri, and the president of Brazil, Dilma Rousseff. Burmese opposition leader Aung San Suu Kyi is among 13 Nobel peace laureates and hundreds of parliamentarians all over the world who have sent messages of support to the imprisoned 30 and called for their release.

Greenpeace has seen its own membership swell and messages of support and donations flood in. Even the Russian president, Vladimir Putin, said that he didn't think the 30 were "pirates".

Sunday will see other pressure groups in cities around the globe take part in protests in solidarity with the environmental charity.

John Sauven, the executive director of Greenpeace UK, told the Observer that frustrations were rising over the slowness and ineptness of the legal process.

"Two weeks ago, we were told that the Arctic 30 detainees would have the piracy charges dropped. This has not happened. One week ago, we were told the detainees would be moved to St Petersburg. This has not happened," he said.

"While I am fully prepared to believe that the confusion and delay is an inevitable consequence of a bureaucracy trying to run an impossible case against innocent people, from the perspective of the friends and families this feels like psychological warfare. I can only imagine the pressure detainees must be under in the detention cells in Murmansk.

"Communication is absolutely vital, even if it has to happen by post. The letters we've received from the 30 have energised Greenpeace supporters across the world. They have reminded us that the activists and journalists in Russia are not pirates, or celebrities, or politicians, or hooligans. The activists were just ordinary people who wanted to protect a threatened wilderness from industrialisation by oil corporations.

"But the really vital communication is what we can get through the other way. In a Murmansk jail cell in winter, enmeshed in a Russian legal system and facing an uncertain future, the one thing you really don't want is to be forgotten. So I'd like to thank everyone who wrote a letter or an email or signed a petition. If you haven't, because you weren't sure it would help, please do. It will. It's not just their freedom that is at stake but the Arctic, its wildlife and people, and the stability of our climate."

In his letter, published here to mark the 53rd day of the Arctic 30's detention, Ball wrote: "I'm not a pirate and I'm not a hooligan. Can I come home now?"