By Roland Buerk
BBC News, Tokyo
Peter Bethune could be charged with trespassing and face prison
An activist from New Zealand has been arrested by Japan's coastguard after he boarded a Japanese whaling ship in the Southern Ocean last month.
Peter Bethune said he had boarded the ship intending to make a citizen's arrest of the Japanese crew.
Instead, the Shonan Maru 2 immediately set sail for Japan with him on board.
He is a member of the Sea Shepherd Conservation Society, which has been trying to disrupt the annual hunt of the Japanese whaling fleet.
Scores of camera crews and photographers waited on the quayside as the whaling ship sailed into Tokyo bay with the anti-whaling activist on board.
Footage from the Institute of Cetacean Research - first broadcast 6 Jan 2010
Nationalist protestors were carrying Rising Sun flags and placards branding Peter Bethune an "eco-terrorist".
After the Shonan Maru 2 docked, Japanese coastguard officials went on board and arrested him.
He had been detained on the ship as it sailed back to Japan after he boarded it from a jet-ski in the Southern Ocean last month.
His intention was to perform a citizen's arrest on the Shonan Maru 2's captain for what he said was the attempted murder of his crew, and demand compensation.
Mr Bethune was in command of a Sea Shepherd hi-tech stealth boat when it was sliced in two in a collision with the ship as anti-whaling activists clashed with the fleet.
He could now be charged with trespassing on a vessel, and if convicted, face a fine or prison.