TOKYO — The Japanese Coast Guard on Friday arrested an anti-whaling activist from New Zealand who had boarded a whaling ship in the southern Antarctic last month.

Peter Bethune, a member of the Sea Shepherd Conservation Society, was brought back to Tokyo by the whaling ship, the Shonan Maru 2, after he boarded it without permission on Feb. 15. Coast Guard officials were waiting for him at the docks in Tokyo, along with a throng of Japanese reporters and television crews.

Mr. Bethune, 44, was being held in Coast Guard custody in Tokyo, said a guard spokesman, Tomoyuki Suzuki, who added that Mr. Bethune had been formally charged with “vessel invasion.” A Coast Guard investigation was under way, he said, and it was expected that Mr. Bethune would be transferred to police custody on Sunday.

Japanese media reports suggested that Tokyo intends to use Mr. Bethune’s arrest as a warning to Sea Shepherd to end its confrontations on the high seas with Japan’s whaling fleet.