The Sea Shepherd Conservation Society said Paul Watson is due back in Frankfurt court Wednesday to find out if the extradition request will be allowed.
Watson appeared in court Monday but was remanded into custody while authorities confirmed whether Costa Rica still intends to proceed with the request.
That confirmation has since been received and the court could issue a preliminary arrest for the Toronto-born activist at the upcoming hearing, said Sea Shepherd spokesman Peter Hammarstedt.
That would allow German authorities to hold Watson for 90 days while the extradition process officially begins.
"There's no way he will get a fair trial in Costa Rica, that's just not going to happen," Hammarstedt said in a phone interview from Frankfurt.
"Our biggest hope right now is that we get the federal minister of justice in Germany to intervene," said Hammarstedt, who is asking supporters to contact the minister's Berlin office.
Officials have the power to dismiss cases they believe are politically motivated, "and we have strong arguments for that," he said.
Watson is wanted in Costa Rica for allegedly endangering a fishing boat while filming a documentary in 2002.
He was arrested Saturday at Frankfurt Airport on an international arrest warrant issued by the Central American country and is being jailed in isolation.
The alleged incident took place in Guatemalan waters, when the U.S.-based group said it encountered an illegal shark finning operation run by a Costa Rican ship.
The crew accused Sea Shepherd of trying to kill them, but the organization said it simply told them to face prosecution.
The filmmaker behind the documentary came to Watson's defence Tuesday, saying there "was no attempt to harm any human life."
"It was an accidental collision," Rob Steward, who also heads the environmental group United Conservationists, said in a statement.
The matter has drawn attention in Costa Rica as well, with at least one opposition member calling for Watson's release.
Sea Shepherd was formed in 1977 and employs polarizing tactics to prevent whaling and other practices it believes threaten the world's oceans.
-by Paola Loriggio in Toronto