The founder anti-whaling group Sea Shepherd Conservation Society remains in custody in Germany after appearing in court on an extradition warrant.
Paul Watson's lawyer says the veteran campaigner is shocked by the Frankfurt court's decision to keep him detained while the extradition request is being considered, the ABC reports.
Costa Rica has issued the warrant, accusing Mr Watson of endangering a fishing boat in 2002.
Sea Shepherd has labelled the charges as nonsense. It says the boat had been taking part in an illegal shark finning operation.
The practice involves catching sharks, slicing off their fins and tossing the sharks back into the sea, sometimes barely alive.
Mr Watson was passing through Frankfurt on his way to France to attend a conference when he was arrested.
Sea Shepherd is best known for disrupting Japan's annual whale hunt.