The plant was "started at 19:02" (1002 GMT), Tokyo Electric Power Company (TEPCO) spokesman Tatsuya Matoba told AFP of the Kashiwazaki-Kariwa plant in Niigata prefecture.

The regional governor approved the resumption last month, although public opinion remains sharply divided.

On Tuesday, a few dozen protesters -- mostly elderly -- braved freezing temperatures to demonstrate in the snow near the plant's entrance, whose buildings line the Sea of Japan coast.

"It's Tokyo's electricity that is produced in Kashiwazaki, so why should the people here be put at risk? That makes no sense," Yumiko Abe, a 73-year-old resident, told AFP.

Around 60 percent of residents oppose the restart, while 37 percent support it, according to a survey conducted in September.

TEPCO said Wednesday it would "proceed with careful verification of each plant facility's integrity" and address any issues appropriately and transparently.

Kashiwazaki-Kariwa is the world's biggest nuclear power plant by potential capacity, although just one reactor of seven was restarted.

The facility was taken offline when Japan pulled the plug on nuclear power after a colossal earthquake and tsunami sent three reactors at the Fukushima atomic plant into meltdown in 2011.

However, resource-poor Japan now wants to revive atomic energy to reduce its reliance on fossil fuels, achieve carbon neutrality by 2050 and meet growing energy needs from artificial intelligence.

Prime Minister Sanae Takaichi has voiced support for the energy source.

Fourteen reactors, mostly in western and southern Japan, have resumed operation since the post-Fukushima shutdown under strict safety rules, with 13 running as of mid January.

Kashiwazaki-Kariwa is the first TEPCO-run unit to restart since 2011. The company also operates the stricken Fukushima Daiichi plant, now being decommissioned.

Nearly 15 years after the disaster, "the situation is still not under control in Fukushima, and TEPCO wants to revive a plant? For me, that's absolutely unacceptable", said Keisuke Abe, an 81-year-old demonstrator.

- 'Anxious and fearful' -

The vast Kashiwazaki-Kariwa complex has been fitted with a 15-metre-high (50-foot) tsunami wall, elevated emergency power systems and other safety upgrades.

However, residents raised concerns about the risk of a serious accident, citing frequent cover-up scandals, minor accidents and evacuation plans they say are inadequate.

"I think it's impossible to evacuate in an emergency," Chie Takakuwa, a 79-year-old resident of Kariwa, told AFP.

On January 8, seven groups opposing the restart submitted a petition signed by nearly 40,000 people to TEPCO and Japan's Nuclear Regulation Authority.

The petition said the plant sits on an active seismic fault zone and noted it was struck by a strong quake in 2007.

"We can't remove the fear of being hit by another unforeseen earthquake," it said.

"Making many people anxious and fearful so as to send electricity to Tokyo... is intolerable."

Before the 2011 disaster -- which killed around 18,000 people -- nuclear power generated about a third of Japan's electricity.

- String of scandals -

Japan's nuclear industry has also faced a string of scandals and incidents in recent weeks, including data falsification by Chubu Electric Power to underestimate seismic risks.

At Kashiwazaki-Kariwa, TEPCO said Saturday that an alarm system failed during a test.

"Safety is an ongoing process, which means operators involved in nuclear power must never be arrogant or overconfident," TEPCO President Tomoaki Kobayakawa said in an interview with the Asahi daily newspaper.

Japan is the world's fifth-largest single-country emitter of carbon dioxide after China, the United States, India and Russia, and is heavily dependent on imported fossil fuels.

Nearly 70 percent of its electricity in 2023 came from coal, gas and oil -- a share Tokyo wants to slash to 30-40 percent over the next 15 years as it expands renewable energy and nuclear power.

Under a plan approved by the government in February, nuclear power will account for around a fifth of Japan's energy supply by 2040 -- up from around 8.5 percent in the fiscal year 2023-24.

Meanwhile Japan still faces the daunting task of decommissioning the Fukushima plant, a project expected to take decades.

kh-hih/mjw

TEPCO - TOKYO ELECTRIC POWER

CHUBU ELECTRIC POWER