New Zealand anti-whaling activist Peter Bethune told a Tokyo court he had thought the acid he threw on to a Japanese ship was only what its common name suggested it was - rancid butter.

Bethune told the court that he had thought butyric acid, which others in Sea Shepherd called "rancid butter", was indeed only rancid butter.

He would never have launched acid bottles on to whaling ships had he known they could cause injury, he said.

Some of it had got on to his hand with no noticeable pain, swelling or any other effects, he said.

Bethune also denied that he had cheered and celebrated after one bottle hit the Japanese deck, as the court heard last week.

The knife he had taken on-board the Japanese ship was a gift from his family - and that was why he had kept it instead of throwing it into the ocean, he said.

"A knife is useful for cutting rope or wire. As a captain it is a most appropriate gift," the Sankei Shimbun quoted him as saying.

When Bethune was asked why he had hid the knife once on-board the Japanese ship, a spectator - a man in his 40s - yelled out: "Fascist! Terrorist!"

Once the commotion settled, Bethune said he had hid the knife because he thought it would become a problem if he was found with it.

Bethune rules out Southern Ocean activity

Bethune shed tears when questioned about his sunken trimaran, the Ady Gil, reports a Japanese newspaper.

Bethune also told the court he would not be returning to the Southern Ocean for protests.

"I have dissatisfactions that whaling takes place in an ocean that's like New Zealand's backyard," Bethune said, according to news agency Jijicom.

"Anti-whaling groups are showing recorded videos in opposition, but those are being ignored."

The Sankei Shimbun said Bethune was wearing a black suit and white shirt, and was flushed with nervousness.

When the defence questioned him about his lost boat Ady Gil, Bethune shed tears as if reminiscing, the newspaper said.

Bethune suggests Japanese water cannons contained acid

On the day the acid bottle had been hurled, Sea Shepherd was planning to launch a helicopter for filming and there was a need to distance the Japanese ship and its water cannons, Bethune told the court, according to the Sankei Shimbun.

He also told the court that many Sea Shepherd crew had been injured by water cannons, including one person in the eye.

He had launched four or five acid bottles at the Japanese ship and one of them had hit the deck, he said.

He had seen three people in the lowermost deck when he fired the bottles, Bethune said.

"There was no one where I aimed so I fired [the bottles]. Maybe people were hurt because there were chemicals in the Japanese water cannons," said Bethune, according to TV news broadcaster FNN.

When Bethune took the stand, Judge Tawada told the court: "The crux of this case is whether an assault charge can be established. Any statement unrelated to this aim has no place, including espousing your group's philosophy. In the case unrelated statements are made, questions will be restricted."

Bethune labelled a "fascist" in court

NZ anti-whaler Peter Bethune has taken the stand at his trial in Tokyo with a warning from the judge and a court spectator jeering him as a "fascist".

Bethune is facing his third day in court for five criminal charges after boarding a Japanese whaling vessel earlier this year.

Today, he continued to assert he had no criminal intentions of assault, the Sankei Shimbun reported.

"I didn't think there would be a problem even if someone was splashed with acid," the newspaper reported in Japanese that Bethune had said in court.

"I aimed for a spot where I couldn't see any crewmen."

As Bethune prepared to be questioned in court, presiding judge Tawada warned him that "this is not the place to espouse your group's philosophy".

At another point during the first hour of today's hearing, security had to control a man in the court who yelled out "fascist" to Bethune, the Sankei Shimbun said.

Japanese newspaper the Asahi Shimbun reported Bethune's interpreted comments that were told in court.

The acid was thrown "so a Sea Shepherd filming helicopter would not get hit by water [from cannons]", Bethune said.

"It was to prevent the Shonan Maru from approaching the helicopter's departure point."

Bethune also told the court his motivations for taking part in anti-whaling disruptions.

"Anti-whaling attitudes were growing in places like New Zealand and Australia, and I thought I should also act," the Asahi Shimbun reported Bethune as saying.

"I want to see harsh punishment" - prosecutor

Japanese reports have focused on Bethune using a launcher to hurl a glass bottle full of butyric acid at the Japanese ship, causing injury to a crewman.

On the first day of trial, on Thursday, the court heard that the man had suffered light chemical burns needing a week to heal.

A video of the incident was shown, taken from the Japanese ship's mast with a running commentary from a crewman.

During the video, Bethune leaned forward and clenched his fists on his knees, looking a little restless, said the Sankei Shimbun.

"A rubber boat is approaching. We are responding by releasing water [through water cannons]. They're throwing something at the ship. I don't know what it is. They're turning the boat and throwing something. They're approaching from the right," a crewman said in the video.

"He's [Bethune is] preparing something like a launcher. The launcher has fired. The Ady Gil is moving away," he said.

According to the Sankei Shimbun's transcript, voices could then be heard saying "are you alright?" and "something's fallen down".

"Something's blown up. A crewman seems to have been covered with acid. We've had something filled with acid thrown on board. It seems to have hit a crewman. I will go downstairs and confirm. Ending transmission," the crewman said.

The prosecutor then told the court that Bethune was heard cheering and seen celebrating after the shot.

Bethune told the court through an interpreter that he had had no intention of hurting anyone. He admitted he had thrown the acid bottle on to the ship, but said there were many circumstances surrouding the incident that would be revealed later.

He contested the assault charge but admitted four other charges including trespassing and carrying a knife.

In the second day of the trial, on Friday, the injured man testified, saying he had felt a burning pain in his right eye and thought he would go blind.

In response to questions from the prosector, he said: "The accused [Bethune] is not at all apologetic. I want to see harsh punishment so he can apologise."

Bloggers' reactions
'Chemist at university' took issue with Bethune claiming he did not know the acid he had hurled was dangerous. But the blogger reserved most of his ire for comments made by Sea Shepherd captain Paul Watson.

"I'm not a right-wing nationalist, and I do not think of branding this activist as 'anti-whaling' and 'anti-Japan' for political purposes.

"Simply, I cannot sympathise with people who take illegal, dangerous actions that hurt other people. They are terrorist acts and should be censured.

"I thought [Sea Shepherd captain] Paul Watson might come to Japan prepared to be arrested, but he is only barking from a safe place. He gives the impression that he thinks: 'Bethune is disposable and I have plenty more activists'."

'Guriken' also had problems with Bethune contesting his assault charge despite admitting he had launched a bottle of acid at the Japanese ship.

"Forget about some tedious chemical like butyric acid, any primary school kid understands that launching a bottle full of even water at people is dangerous. But apparently for Bethune it was a perplexing question that he found hard to make a judgment on (strained laugh)."