Pete Bethune's defence in court that he thought firing acid bottles at a Japanese whaling ship was safe has sparked widespread reaction in Japan.

Bethune spoke at Tokyo District Court yesterday, facing up to 15 years in jail for five criminal charges including assault and trespass, incurred after clashes with whalers earlier this year.

"He claims he was told the acid was harmless. What he's saying is absurd. If he didn't want any harm, they should never have used chemicals," said blogger Mirai Shikou.

In response to questions from prosecutors and judges, Bethune explained that he had aimed the bottles at an area of the ship where there had been no crew.

He had filled glass bottles with butyric acid because of its strong smell in the hope of distracting the whaling vessel while activists launched a camera helicopter, he said.

A judge asked if he thought the acid was "safe" like lemon juice.

Bethune said yes, even if it would hurt when splashed in someone's eye.

Another blogger, Domon, claimed that scientific tests had shown 2ml of the pure acid killed 50 per cent of rabbits.

"We can't take this toxicity lightly," he said.

"I don't know how concentrated the acid [used by Bethune] was, but even if it was believed the acid was harmless, there needs to be responsibility when it was deliberately thrown and injury resulted."

Many were also sceptical of his testimony that he would be retiring from anti-whaling activities in the Southern Ocean.

"He's saying he won't go to the Southern Ocean, but will he go to sea elsewhere? Really? Retiring?" wrote Kankin Kun.

"If he's to be forgiven because he's retiring, we'd need to forgive gangs too. Plus, only an idiot would believe he's quitting just because he said so," said Mirai Shikou.

Others took to a discussion of the wider whaling issue.

"There are whales we need to protect. But there also seem to be whales that can be commercially caught in balance with nature," commented one person.

"Personally, I'd like to see whale meat, which has difficulty being understood globally, get wider acceptance around the world."

An online group calling itself "the committee to think about the Sea Shepherd problem" wrote online that Bethune had been used by Sea Shepherd, after his statement yesterday that he still owed money on his sunken boat, the Ady Gil.

"In other words, did Sea Shepherd just use the Earth Racer's fame [the former name of the Ady Gil], to get footage of New Zealand hero Bethune's courageous fight with Japanese whaling ships?"

The group made an invitation to Bethune to stay in Japan and work for co-operation: "Please use the whaling issue as an opportunity to start a second life in Japan and become a bridge between Japan and New Zealand, to work out issues of marine environmental protection and civilian involvement."

Meanwhile, blogger Hiroyuki Seto followed a group of right-wing protesters from an anti-Chinese xenophobic protest - where placards read "Sino are enemies of humanity" - straight to Bethune's trial at Tokyo District Court.

There, protesters switched to banners that read: "Terrorist group Sea Shepherd supporters New Zealand: Don't be conceited!"

Bethune faces a final day in court on June 10, when closing statements will be made.