Criticism of the Japanese government's handling of the radiation crisis at a nuclear power plant has increased after an adviser quit in protest over what he claimed were unsafe, slipshod measures.

Toshiso Kosako, a professor at the University of Tokyo's graduate school and an expert on radiation exposure, announced late yesterday that he was stepping down as a government adviser.

Prime Minister Naoto Kan appointed Prof Kosako after the magnitude-9.0 earthquake and tsunami which struck north-eastern Japan on March 11.

The disaster left 26,000 people dead or missing and damaged several reactors at the Fukushima Dai-ichi nuclear power plant - triggering the world's worst nuclear crisis since Chernobyl in 1986.

In a tearful news conference, Prof Kosako said he could not stay and allow the government to set what he called improper radiation limits of 20 millisieverts an hour for elementary schools in areas near the plant.

"I cannot allow this as a scholar," he said. "I feel the government response has been merely to bide time."

Prof Kosako also criticised the government as lacking in transparency in disclosing monitoring of radiation levels around the plant, and as improperly raising the limit of radiation exposure levels for workers at Fukushima Dai-ichi, according to the Kyodo News agency.

The prime minister defended the government's response as proper.

"We welcome different views among our advisers," Mr Kan told parliament today in response to an opposition politician's questions.

A government advisory position is highly respected in Japan, and it is extremely rare for an academic to resign in protest of a government position.

The science and education ministry has repeatedly defended the 20-millisievert limit as safe, saying that efforts are under way to bring the limit down to 1 millisievert. Some people have expressed concerns, noting that children are more vulnerable to radiation than adults.

Japan, which has 54 nuclear reactors, has long been a major proponent of atomic power, constantly billing its technology as top-rate and super-safe.

Japan's government has also been trying to make deals to build nuclear power plants in other countries, although such attempts are likely to fall flat after the Fukushima Dai-ichi accident.

As the only country in the world to suffer an atomic bombing, as it did at Hiroshima and Nagasaki during the Second World War, Japan has long had a powerful anti-nuclear movement, and such protests have become louder recently.

About 450 protesters gathered in Tokyo's Yoyogi Park today, beating drums, shouting "No more nukes" and holding banners which read "Electricity in Tokyo, sacrifice in Fukushima".

"We knew all along nuclear power was dangerous. I just didn't know how to express myself," said one of the protesters, 50-year-old Yoshiko Nakamura, who was taking part in her second demonstration in two weeks.

"This is a great opportunity to send a message and voice my fears."

Such demonstrations have become more frequent, including during the Golden Week holidays, which continue through the weekend and next week.