Costa Rican President Laura Chinchilla said Wednesday that anti-whaling activist Paul Watson would have a fair trial if extradited to the Central American country following his arrest in Germany.

"If Paul Watson is extradited to Costa Rica he will have legal proceedings that strictly keep to constitutional principles and the international standards which have to be applied in this type of case," she said.

Chinchilla, speaking at a press conference with German President Joachim Gauck during a visit to Berlin, stressed that her country had a "completely independent justice" system.

Watson, 61, the leader of the Sea Shepherd organisation noted for its muscular attacks on Japanese whalers, was arrested at Frankfurt airport in western Germany on May 14 and detained for a week before being released on bail.

German authorities are deciding whether he can be extradited to Costa Rica on charges stemming from a high-seas confrontation over shark finning in 2002. He is accused of "putting a ship's crew in danger".

He said in a telephone interview with AFP Tuesday that he had "no reason to believe that the Costa Rican legal system would not give me a fair trial".

"My concern is not for the judicial system, but for the reality that the shark fin mafia of Costa Rica has a price on my head and a Costa Rican prison would provide an excellent opportunity for someone to exercise this lethal contract against me," he said.

"We have cost the shark finners a great deal of money over the last two decades and they want their revenge. I would need absolute assurance that the Costa Rican authorities would not place me in a position to jeopardise my safety when I return to Costa Rica to prove my innocence in court."

Sea Shepherd claims it was escorting an illegal shark finning ship back to port when the crew falsely accused the organisation's members of trying to kill them.

Watson said it was unusual that an extradition order should be issued for "a relatively minor offence, where no one was injured and no property damaged."

He suggested that Japan might be "putting pressure" on Germany to carry out the order.

"It may be more than coincidental that the extradition order was put out in October 2011 at around the same time that the Japanese brought civil charges against us -- and lost -- in a Seattle court," he said.

On the sidelines of Chincilla's visit to the German capital, Watson joined several hundred of his supporters who were shouting "Free Paul Watson!" for a protest in front of the Victory Column in central Berlin.

He had told AFP he would have to be back in Frankfurt by 5:00 pm to report to the police.

Shepherd also vowed Tuesday that Sea Shepherd's campaigns against would go on without him and voiced suspicions that Japan was behind his arrest.

"This is not about me. It is about our oceans and the ever-escalating threat of diminishment of the diversity of life in our seas. It is about the sharks, the whales, the seals, the sea turtles and the fish," he told AFP.

"They hope that by getting me out of the way, they'll shut down our operations. They won't," he said.

Sea Shepherd's ships would continue to "defend sharks in the South Pacific, whales in the Southern Ocean Whale Sanctuary" and dolphins in Taiji, Japan, he added in a statement.

The Canadian national is well known for his pursuit and harassment of Japanese whaling boats off Antarctica, which in recent years has significantly reduced the number of animals slaughtered.