Japan's prime minister at the height of the Fukushima Daiichi nuclear crisis has admitted he often felt "helpless" during the early days of the disaster, adding that the facility's triple meltdown had brought the country close to "national collapse".

Speaking at a parliamentary inquiry into the handling of the incident on Monday, Naoto Kan urged Japan to abandon nuclear power, as the industry attempts to bring closed reactors back into operation.

Kan, who resigned last September amid criticism of his handling of the crisis, has become one of Japan's most vocal opponents of nuclear power.

His successor, Yoshihiko Noda, has argued in favour of restarting reactors deemed safe to avoid electricity shortages this summer.

In comments that were among the most eagerly awaited in the six-month public enquiry, Kan said inadequate explanations from nuclear officials drafted in to brief him on the situation at the plant had only increased his sense of fear.

He reserved his harshest criticism for officials from the nuclear and industrial safety agency, the country's main regulatory body.

"I was frightened and felt helpless," he told the panel, set up after a previous government investigation was criticised as timid. "You can't expect a nuclear expert to be prime minister or a cabinet minister, so we need top regulatory officials to provide expertise and help us. We didn't have those people."

He pointed to fundamental weaknesses in Japan's readiness to deal with a disaster as serious as the 14-metre tsunami that caused three of Fukushima Daiichi's reactors to go into meltdown.

A law to improve the country's ability to address a nuclear disaster introduced after a fatal accident at a uranium processing plant in Tokaimura in 1999 was found wanting, he said.

"Everything anticipated in the law was inadequate, and we had to endure all kinds of trouble that we could have done without," he said.

Assessments of the aftermath of Fukushima tell a story of confusion at the site, and a lack of communication between Tepco, safety officials and the government. The accident has fuelled criticism of the "nuclear village" – regulators, utilities, bureaucrats and pro-nuclear scientists – who, Kan said, were again working together to restart reactors despite local concerns over safety.

Long-awaited reform of the safety agency has been held up by opposition parties, which have demanded that the new body be given more independence from the nuclear agency.

Kan denounced close ties between power companies and their regulators as "the root of the illness" whose symptoms became most apparent during the Fukushima crisis.

But he added: "The nuclear accident was caused by a nuclear plant that operated as national policy, so I believe the biggest portion of blame lies with the state."

His testimony shed light on the disarray at the heart of the small group of officials tasked with leading Japan's emergency response, but it failed to resolve accusations that Kan's actions in the immediate aftermath had worsened the situation.

He said Tepco and the nuclear safety agency had hidden key details from him in the days after 11 March, adding that he had been as open as possible with the public based on the information he had been given.

Kan said he had feared further meltdowns that could result in the evacuation of Tokyo – a metropolitan area of more than 30 million people. Deserting the capital, he added, would have brought the government to a standstill and led to "a collapse of the nation's ability to function".

He defended his demand that Tepco officials remain on duty after being told that the firm planned to evacuate its staff from the plant. The company has claimed that it never intended to abandon the facility, but would have retained a core of engineers to prevent the situation from spiralling out of control.

Kan also defended his hastily arranged visit to the plant the day after the tsunami, dismissing accusations that his presence hindered the operation to prevent catastrophic meltdown during its most critical phase.

Kan said he had decided to talk to the plant's manager, Masao Yoshida, directly because officials in Tokyo had provided only sketchy information.

He disputed Tepco's claims that he had ordered the utility to stop injecting seawater into the stricken reactors to keep them cool. Yoshida, who took early retirement last year after being diagnosed with cancer, ignored the order and has been credited with preventing an even worse accident.

Kan's opposition to nuclear power has not weakened since his resignation. Citing the risk of a "national collapse" from a similar accident in future, he said the Fukushima crisis had convinced him that "the best way to make nuclear power plants safe is not to depend on them, but not to have them at all".

Noda, meanwhile, is expected to decide soon whether to order the restart of two reactors in Oi, western Japan, amid fears of a power shortfall during the height of the summer.