A spokesperson for Japanese whalers in the Southern Ocean says a New Zealand protester who climbed aboard one of their ships committed an act of piracy.
Pete Bethune was the skipper of anti-whaling vessel the Ady Gil, which sank in a collision with one of the Japanese fleet in January.
Mr Bethune says he boarded the Shonan Maru 2 on Monday morning to carry out a citizen's arrest of the captain and present him with a $3 million bill for damage to the vessel.
A spokesperson for the whalers, Glen Inwood, dismissed Mr Bethune's protest and says his actions are illegal.
Mr Inwood says Mr Bethune cut himself while getting on the ship and is being treated by a doctor.
He is being kept in a room on his own while the Japanese authorities discuss their next step.
A spokesperson for the anti-whaling group Sea Shepherd Conservation Society, Locky Maclean, says Mr Bethune has refused to leave the ship until his demands are met.
New Zealand Prime Minister John Key says calm heads need to prevail on both sides of the argument. He says the real risk in this situation is if something goes tragically wrong.
Japanese television network NHK reports Mr Bethune is to be handed over to Japanese maritime authorities.