group Sea Shepherd, Paul Watson, has appeared in court in Germany.
The marine conservationist faces extradition to Costa Rica over charges arising from a clash with a ship that was fishing for sharks.
The confrontation occurred in 2002 in Guatemalan waters.
An official from the prosecutor's office said Mr Watson was accused of using his vessel to intimidate another, putting the crew at risk.
Guenter Wittig said Mr Watson was currently in temporary custody and a judge would decide whether to formally place him in detention pending extradition.
Sea Shepherd is a controversial direct action group best known for disrupting Japan's annual whale hunt.
In the past there have been collisions between its vessels and the whaling fleet, and its activists have also boarded Japanese vessels.
The group said the German warrant related to an "alleged violation of ships traffic" which took place a decade ago when it "encountered an illegal shark finning operation" - referring to the practice of catching sharks, slicing off their valuable fins and returning the shark to the water where it will usually die.
On order of the Guatemalan authorities, the group says, it instructed the crew of the Costa Rican vessel in question, the Varadero, to head back to port to be prosecuted.
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