The founder of conservation group Sea Shepherd, which annually disrupts Japan's whale hunt, has been arrested in Frankfurt for extradition to Costa Rica, the organisation said Monday.

Paul Watson, a 61-year-old Canadian national, is in a German jail after being detained over the weekend on charges stemming from a high seas confrontation over shark finning in 2002.

"I am currently being held in Frankfurt on charges from Costa Rica," tweeted Watson, who counts Hollywood stars Sean Penn, Pierce Brosnan, Martin Sheen and Sean Connery among Sea Shepherd's board of advisors.

"Court appearance in the morning," he added.

A report in Australia, citing Costa Rican media, said the activist, who claims to be a co-founder of Greenpeace, also faced an outstanding warrant for attempted murder during the same incident.

"The German police have said that the warrant for Captain Watson's arrest is in response to an alleged violation of ships traffic in Costa Rica, which occurred during the filming of 'Sharkwater' in 2002," Sea Shepherd said.

The specific "violation of ships traffic" incident took place in Guatemalan waters when Sea Shepherd encountered an illegal shark finning operation, run by a Costa Rican ship called the Varadero, it added in a statement.

"On order of the Guatemalan authorities, Sea Shepherd instructed the crew of the Varadero to cease their shark finning activities and head back to port to be prosecuted," the group said.

It claimed that while escorting the Varadero back to port, the tables were turned and a Guatemalan gunboat was dispatched to intercept the Sea Shepherd crew.

"The crew of the Varadero accused the Sea Shepherds of trying to kill them, while the video evidence proves this to be a fallacy," said the group, which was set up in 1977 to campaign against the slaughter of ocean wildlife.

"To avoid the Guatemalan gunboat, Sea Shepherd then set sail for Costa Rica, where they uncovered even more illegal shark finning activities in the form of dried shark fins by the thousands on the roofs of industrial buildings."

Watson, who has become internationally renowned for his daring and aggressive approach to conservation, was being assisted while in prison by members of the European Parliament Daniel Cohn-Bendit and Jose Bove, Sea Shepherd added.

"Our hope is that these two honourable gentlemen can set Captain Watson free before this nonsense goes any further," the group said.

"Conservationists around the world maintain hope that the Costa Ricans will drop the charges against Captain Watson."

Sea Shepherd is best known for its annual pursuit of the Japanese whaling fleet in Antarctica, using increasingly militant ways to halt the hunt, including boarding the Japanese vessels.

This year, after setting off from Australia, the group hurled stink bombs at the boats on the high seas and used ropes to try to tangle their propellers in a series of exchanges which saw the whalers retaliate with water cannon.

The whaling fleet killed less than a third of the animals it planned to because of the sabotage attempts.