The battle between Japan and anti-whaling activists will be played out in a Tokyo courtroom today with the start of the trial of Peter Bethune, who was arrested after clambering aboard a Japanese whaling ship earlier this year.

Bethune, of the Sea Shepherd marine conservation group, faces five charges: trespass, assault, illegal possession of a knife, destruction of property and obstruction of business. These crimes carry a maximum penalty of 15 years in prison.

Dan Harris, a Sea Shepherd lawyer who visited Bethune in detention yesterday, said the activist was "unbelievably well. He's in really good spirits, very healthy and excited about having the opportunity to explain himself to the world."

Harris said prosecutors were expected to demand a jail sentence of between two and a half and three years. "We are far more optimistic," he said. "Our focus is on getting Pete out of Japan as quickly as possible.

"He is very proud of his role in reducing the whale catch and spreading the word about what is really happening in the Southern Ocean. He feels that whatever sentence he gets, it will have been worth it. He has no regrets."

A verdict is expected on 10 June, with sentencing up to four weeks later, Harris said.

Bethune, a 45-year-old New Zealander, was arrested after jumping on to the Shonan Maru 2 in February to protest against the sinking the previous month of the Ady Gil, Sea Shepherd's powerboat. The boat, which Bethune skippered, was sliced in two during a collision with the Japanese vessel and later sank.

Sea Shepherd had been using the Ady Gil to pursue and harass the whaling fleet during its annual "research" whaling expedition to the Antarctic. The group regards Bethune as a political prisoner.

"We are taking the trial very seriously and have confidence in the Japanese system to act fairly and send Pete home," said Jeff Hansen, Sea Shepherd's director for Australia. "It is horrendous what he's been through, so we hope Japan comes to its senses and exercises good judgment."

Under cover of darkness, Bethune leaped on to the harpoon boat from a jetski in an attempt to carry out a citizen's arrest of the captain and present him with a $3m (£2.08m) bill for the loss of the powerboat. He was held on board and arrested on 12 March when the ship returned to Japan. He has been held at Tokyo detention centre ever since.

In an interview with New Zealand's Sunday Star-Times, Bethune said he stood "a 95% chance" of being convicted and serving a long prison term. He said he had been escorted by more than 100 security guards when he was taken back to the Shonan Maru 2 to re-enact the boarding: "I had a hood over me, like I'm a psychopathic killer. It was bizarre."

Hansen urged Australia and New Zealand to increase pressure on Japan to end its culls in the Southern Ocean whale sanctuary and to secure Bethune's release.

"I don't think either government has done nearly enough for Pete," he said. "The Australian government promised two years ago to stop the whale hunts, yet we still have volunteers like Pete putting their lives on the line.

Recent years have been marked by dangerous clashes on the high seas between whalers armed with water cannon and Sea Shepherd members, whose tactics include hurling parcels of rancid butter on to the decks of whaling ships.

Japan claims Bethune was part of a butyric acid attack that left one whaler with burns to his face. It has also issued an international warrant for the arrest of Paul Watson, Sea Shepherd's founder, for endangering the lives of whaling crews by masterminding attacks on their ships.

Watson described Bethune as a "prisoner of war", and demanded that the captain of the Shonan Maru 2 be questioned over the Ady Gil sinking.

The trial opens as the International Whaling Commission prepares to meet in Morocco to discuss a controversial proposal that would allow Japan, Norway and Iceland to hunt a limited number of whales commercially in return for reducing their catch over the next decade.

The 1986 ban on commercial whaling allows Japan to kill whales for "lethal research" and sell the meat on the open market. The leader of the whaling fleet blamed harassment by Sea Shepherd activists after his crew returned last month with about half its target catch of 935 whales.