Two Canadians, Montrealer Alexandre Paul and Paul Ruzycki, of Port Colborne, Ont., were among those arrested on the Greenpeace ship Arctic Sunrise on Sept. 18.
On Friday, the organization released a handwritten letter in which Paul details his plight in prison.
He said he's isolated from the other detained activists. Their home for the past five weeks has been a prison in Murmansk, a port city near Norway and Finland.
"The organization has been good to us providing extra food and clothes. The weather is turned to winter, the days are getting shorter," Paul wrote.
"As a Canadian, you shouldn't worry for me, but have a thought for my colleagues from Brazil, southern (Italy), Argentina and Turkey. It must be cold for them."
Russia seized Arctic Sunrise after a Sept. 18 protest at a Gazprom oil-drilling platform in the Arctic circle. The group on the ship included 28 Greenpeace activists, a Russian photographer and a British videographer.
The 30 aboard were initially charged with piracy but those charges have since been reduced to hooliganism. That charge carries a maximum seven-year prison sentence upon conviction.
Greenpeace has denied any wrongdoing and describes the charges as absurd.
Paul, 35, who has worked with Greenpeace for 15 years, is hopeful Canadians will show their opposition by signing petitions and contacting federal politicians.
He also asks supporters to write to Russian embassies around the globe.
"Tell them this imprisonment is unfair and illegal," Paul said.
"Tell them we are not pirates."
Paul says whatever people decide to do, they should do it peacefully.
"We have nothing against Russia, as a matter of fact; we did this for them and their children," he wrote.
In Paul's home province, the provincial government has urged the federal government to act. Jean-Francois Lisee, the Parti Quebecois government's minister of international relations, is asking for clemency in Paul's case.
On Friday, a spokeswoman for Canada's foreign affairs department, Beatrice Fenelon, says the government is limited in the information it can release about the two men.
"We are aware of the situation involving two Canadian citizens in Murmansk, Russia," Fenelon wrote in an email. "Consular services are being provided to the two Canadian citizens as required."
Paul says he'll remain optimistic.
"I'll do my part. I'll stay strong, I won't despair," Paul wrote. "I will keep the faith. For a better world."