Japan on Tuesday restarted its nuclear power programme after a shutdown triggered by the 2011 Fukushima crisis, as the government pushes to return to a cheaper energy source despite widespread public opposition.

Utility Kyushu Electric Power turned on a reactor at Sendai, about 1,000 kilometres (620 miles) southwest of Tokyo, at 10:30 am (0130 GMT).

The 31-year-old reactor -- operating under tougher post-Fukushima safety rules -- was expected to reach full capacity around 11:00 pm Tuesday and would start generating power by Friday.

Commercial operations would begin early next month, a company spokesman said.

The restart comes more than four years after a quake-sparked tsunami swamped cooling systems and triggered reactor meltdowns at the Fukushima plant, prompting the shutdown of Japan's stable of 50 reactors and starting a pitched battle over the future use of atomic power.

The accident sent radiation over a wide area and forced tens of thousands from their homes -- many of whom will likely never return -- in the worst nuclear accident since Chernobyl in 1986.

Decommissioning of the crippled Fukushima reactors is expected to take decades and compensation expenses -- excluding the cost of the site's cleanup -- now top $57 billion.

Anti-nuclear sentiment still runs high in Japan and television showed protesters scuffling with police in front of the plant, which is on the southernmost main island of Kyushu.

Among the 200 protesters was Naoto Kan, prime minister at the time of Fukushima and now a high-profile anti-nuclear activist. He said the failure of pro-atomic premier Shinzo Abe to cancel the restart "cannot be forgiven".

The resource-poor nation, which once relied on nuclear power for a quarter of its electricity, restarted two reactors temporarily to feed its needs after Fukushima. But they both went offline by September 2013, making Japan completely nuclear-free for about two years.

- 'Safety first' -

The country adopted stricter safety regulations to avoid a repeat of the accident, including more backup prevention measures and higher tsunami-blocking walls in some areas.

"It is important to restart reactors one by one from the perspective of energy security, the economy and measures against global warming, but safety always comes first," Industry Minister Yoichi Miyazawa told reporters..

Yukio Edano, a senior member of the opposition Democratic Party of Japan who criticised Abe for taking a holiday near Mount Fuji on Tuesday, said the restart was ill-advised and "trampled on the grave sacrifice seen in Fukushima".

Strengthened safety measures are key to Abe's bid to get some of about four dozen reactors back up and running. The government wants nuclear power to generate up to 22 percent of Japan's electricity needs by 2030, a lower percentage than before Fukushima.

Power companies that own the reactors are also keen for more restarts after having to import pricey fossil fuels.

Japan's post-Fukushima energy bill skyrocketed as it scrambled to fill the gap left by taking reactors offline, a problem worsened by a sharp weakening of the yen which pushed up the cost of dollar-denominated energy imports.

Several other reactors have been given a safety green light, but battle lines are drawn in many local communities strongly opposed to restarts.

"Abe is not listening to the voice of the people -- he is acting as if he has been given a blank cheque," said Takashi Kato, professor emeritus at Tokyo's Seikei University.

Officials have stressed that any switched-on reactor would operate under much tighter regulations than those that existed before Fukushima. Some will be decommissioned for safety reasons.

"A disaster like that at Tokyo Electric Power's Fukushima Daiichi nuclear plant will not occur" under the new rules, Nuclear Regulation Authority chairman Shunichi Tanaka said in an interview with the Nikkei newspaper published at the weekend.

But Tanaka conceded there was "no such thing as absolute safety" and Japan's people are sceptical as the country remains deeply scarred by the legacy of Fukushima -- although no deaths have been directly attributed to the accident.

The government was strongly criticised for its cosy ties with Fukushima operator Tokyo Electric Power Co, which itself was accused of incompetence in dealing with the 2011 crisis.

Last month a judicial review panel decided that a trio of former TEPCO executives should be indicted, paving the way for the first criminal trial linked to the disaster.

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Tokyo Electric Power