Japan went nuclear-free on Monday after switching off its last operating reactor for inspection, and local media said a restart of any reactors is unlikely at least until early next year.

Kansai Electric Power took offline the No. 4 reactor at its Oi nuclear plant in the western prefecture of Fukui at 1:33 am (1633 GMT Sunday) "without any problems", said a company official.

The move left the world's third largest economy without atomic energy to generate electricity for the second time since the Fukushima nuclear crisis erupted in March 2011.

Nuclear power supplied about one-third of the resource-poor nation's electricity before a tsunami knocked out cooling systems and sparked meltdowns at Fukushima, causing tens of thousands to flee their homes.

Prime Minister Shinzo Abe has openly backed a return to the widespread use of atomic energy, but the public remains divided. Opponents cite safety fears.

Some 8,000 people staged a rally in Tokyo during the weekend against any plan to resume operations. Organisers said another sizable anti-nuclear demonstration will be held in the capital next month.

"Let's preserve the environment in which children can live at ease," Nobel literature laureate Kenzaburo Oe told the rally.

Japan previously was without any nuclear energy in May 2012, when all 50 commercial reactors were stopped for checkups in the wake of the disaster.

But the government last year gave Kansai Electric approval to restart No. 3 and No. 4 reactors at the Oi plant, arguing that nuclear energy was necessary to meet increased electricity demand during the winter.

The reactors were reactivated in July 2012 and resumed full commercial operation the following month, but the No. 3 reactor was shut down earlier this month for a scheduled inspection. The nation's other reactors have remained idle.

Utilities this summer have submitted applications to restart their reactors with the Nuclear Regulation Authority, which has significantly upgraded safety standards since the Fukushima crisis.

Currently the nuclear watchdog is reviewing safety at 12 reactors including No. 3 and No. 4 at the Oi plant, officials said.

But the mass-circulation Yomiuri Shimbun newspaper said procedures for a restart are unlikely to be completed by the end of this year.

It quoted a member of the watchdog body as saying: "We have not even reached a peak of the safety inspection, which will take half a year."

The Nikkei business daily also noted that it may take two to three months to gain consent for a restart from local residents.

Radiation was spread over homes and farmland in a large area of northern Japan when a massive tsunami hit Fukushima on March 11, 2011.

No deaths were recorded as a direct result of the meltdowns, but tens of thousands were evacuated and many remain so. Some areas are expected to be uninhabitable for decades.

Tokyo Electric Power is still struggling to contain leaks of radioactive water at the plant.

It has admitted that some 300 tonnes of mildly contaminated groundwater is entering the ocean every day after flowing under the reactors.