Japanese media have expressed frustration at a NZ activist's anti-whaling protests, with one newspaper accusing him of terrorism and another blaming cultural misunderstanding for the situation.

Sea Shepherd activist Peter Bethune, 44, was arrested in Tokyo on Friday after a Japanese whaling ship he had boarded in the Southern Seas reached port.

Mr Bethune faces up to three years in a Japanese jail for trespassing. Prime Minister John Key has said the New Zealand Government cannot intervene in Japan's legal processes.

The Asahi Shimbun and Mainichi Shimbun, two of Japan's top three national daily newspapers with circulations of about eight million and four million, respectively, have published editorials expressing frustration with the anti-whaling protests.

The Mainichi Shimbun said today criminal proceedings against Mr Bethune should be distanced from the whaling debate and proceed "with calmness" (shukushuku).

The Sea Shepherd's anti-whaling protest had gone beyond what can be considered a proper course, it said, citing protesters disrupting ships with lasers and throwing bottles of chemicals on to them.

"[Their protests] should be seen as acts of terrorism," it said.

"It is not worth repeating, but Japan's research whaling expeditions are lawful actions founded on international treaties. Of course, people are free to express anti-whaling positions. But if they want it to stop, they should protest to the International Whaling Commission. They should follow lawful means."

The Asahi Shimbun, meanwhile, said yesterday that the Sea Shepherd crew were a troubling bunch ("mattaku kommatta renchuu de aru").

It questioned whether it was worth responding to Mr Bethune's provocation, as it would not be worth making him a hero in many country's eyes and letting his arrest be used as fuel for anti-whaling protests.

A misunderstanding that Japanese regularly eat whale meat was a cause of the clashes, it said.

"People who eat other meats censure eating whale as cruel, while Japanese, who in fact rarely eat whale meat, claim that eating whale is Japanese culinary culture. It is a bizarre situation. The problem has grown abnormally by a rising agent called cultural friction."

The Asahi Shimbun said Japan should avoid making the issue a cultural or nationalistic one and instead address anti-whaling views with a level head.

Mr Bethune's arrest follows months of high-seas clashes with the Japanese whaling fleet involving Mr Bethune, who is a member of the Sea Shepherd Conservation Society.

He has been in custody since mid-February, when he boarded the Japanese vessel Shonan Maru II intending to make a citizen's arrest on its captain for what he said was the attempted murder of his six crew.

* This article is based on reporter Michael Dickison's translations of the Asahi Shimbun and Mainichi Shimbu articles.