An anti-whaling activist goes on trial in Tokyo Thursday on charges stemming from months of high-seas clashes between militant environmentalists and Japanese harpoon ships in Antarctic waters.

New Zealander Peter Bethune of the US-based Sea Shepherd Conservation Society was detained more than three months ago and faces five charges in Tokyo District Court that could see him jailed for up to 15 years.

The case will throw a spotlight on whaling, which Japan defends as part of its cultural tradition and carries out under a loophole to an international moratorium that allows lethal "scientific research".

The Sea Shepherd group pursued and harassed Japanese whalers in Antarctic waters for months in the 2009-2010 season, a campaign which both sides say reduced the Japanese cull by several hundred whales.

In their annual clashes, the adversaries regularly trade icy jets from water cannon, while the environmentalists also hurl blood-red paint bombs and rancid butter, or butyric acid, stink bombs at the whaling ships.

Bethune, 45, was the captain of the futuristic kevlar powerboat the Ady Gil, which was sliced in two in a collision with the Japanese fleet's security ship the Shonan Maru II in January and sank soon after.

The next month Bethune scaled the Japanese ship from a jetski with the intent of making a citizen's arrest of its captain for the attempted murder of the Ady Gil's six crew, and to bill him for the sunken vessel.

Instead, Bethune was detained and taken back to Japan, where angry protesters greeted the ship as it docked at Tokyo harbour and he was formally arrested by the Japanese Coast Guard.

He was charged with trespass, vandalism, obstructing commercial activities and carrying a knife when he scaled the ship, as well as with assault for earlier allegedly injuring a whaler with a rancid butter projectile.

Sea Shepherd founder and head Paul Watson, writing on the group's website, has described Bethune as a "political prisoner" and praised him as "the only true samurai presently residing in Japan."

"Captain Peter Bethune was well aware of the possible consequences of boarding the Shonan Maru II," he wrote.

"He chose to do what he thought was right, and like any prisoner of war he will not betray his cause in the face of threats of incarceration and intense interrogation."

Bethune believes "there is a 95 percent chance" he will be convicted and receive a long prison term, he has said in an interview with his country's Sunday Star-Times from Tokyo Detention Centre.

Bethune said he was guarded by a security detail of more than 100 officers when he was later taken back to the Shonan Maru II to re-enact the boarding.

"I had a hood over me, like I'm a psychopathic killer. It was bizarre," he reportedly said.

Bethune's US lawyer Dan Harris has criticised the Japanese authorities for trying to stage "a political show trial".

Japan, which says whaling has been part of its culture for centuries, has taken stern action against the activists, also seeking Interpol's help to arrest Watson for ordering his crew to harass whaling ships.

Japan has accused both Sea Shepherd and Greenpeace, a group which Watson once belonged to, of risking the lives of Japanese whalers.

"What they are doing is nothing like peaceful protests but dangerous terrorist acts that threaten human lives, which should never be forgiven," says the Institute of Cetacean Research, which conducts Japan's whale hunts.

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