Wellington, May 31 NZPA - Anti-whaling activist Pete Bethune on trial in Japan, today denied an assault charge, saying rancid butter stink bombs he threw at Japanese harpoon ships were no more acidic than orange juice.

Bethune, 45, faces five counts of illegally boarding a Japanese whaling vessel.

A member of the Sea Shepherd Conservation Society, Bethune was charged following his boarding of the Japanese whaling fleet's security ship Shonan Maru 2 in the waters of Antarctica earlier this year.

Today he tearfully recounted the day his trimaran, the Ady Gil, collided with the Shonan Maru 2 during confrontations in the Antarctic waters, Agence France Presse reported.

Bethune later boarded the Shonan Maru 2 and presented its captain with a letter seeking $3 million in replacement costs for the Ady Gil, which he captained before it sank after the collision.

Prosecutors have told the court a rancid butter, or butyric acid, stink bomb caused chemical burns to the face of a 24-year-old Japanese crew member during a February 11 clash and also hurt the eyes of several other whalers.

Bethune, who has admitted four charges but not assault, said he threw the bottle thinking it "very unlikely" to hit crew members, and that the butyric acid has the same level of acidity as orange juice.

"It's very smelly, but it's okay if it falls in the water. It's organic and biodegradable and won't harm anyone," he told the court.

Meanwhile, passions on both sides were in clear evidence outside the Tokyo District Court building, where Japanese right-wingers gathered to scream blood-curdling abuse at a group they see as violating Japan's rights and traditions. Police were forced to separate them from a far smaller and less frenetic crowd of Mr Bethune's supporters, the Times newspaper's website reported.

After today's court appearance, the case will be adjourned until June 10 for closing arguments.