Ric O'Barry, the former dolphin trainer for the "Flipper" TV series, said immigration officials told him he wasn't a tourist, the visa he was using to enter Japan, according to his lawyer, Takashi Takano.
He said officials accused O'Barry of having close ties with the anti-whaling group Sea Shepherd, which O'Barry denies.
Immigration officials said it is their policy not to comment on individual cases.
Takano said he appealed the detention, and that the Japanese government will decide on whether to allow O'Barry into the country or deport him. The timeframe for that decision wasn't clear.
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"The Japanese government is cracking down on those who oppose their war on dolphins," O'Barry said in a statement sent by email to The Associated Press through his son, Lincoln O'Barry.
Officials in Taiji, a small fishing village in central Japan, and fishermen have defended the hunt as tradition, saying that eating dolphin meat is no different than eating beef or chicken.
Most Japanese have never eaten dolphin meat, and many say they are horrified by the dolphin-killing, and have joined the campaign against the Taiji hunt.
O'Barry has been stopped and questioned at Japanese immigration before, as well as temporarily taken into custody by local police on the suspicion of not having proper travel documents.
But this is the first time he has been detained in this way. He has the support of high-profile celebrities, including Sting, US Ambassador to Japan Caroline Kennedy and former Guns N' Roses drummer Matt Sorum.