Operations to relaunch a reactor at the Kashiwazaki-Kariwa plant in Niigata province, closed since the 2011 Fukushima disaster, began late Wednesday after it received the final green light from the nuclear regulator despite divided public opinion.

"An alarm from the monitoring system... sounded during the reactor startup procedures, and operations are currently suspended," Takashi Kobayashi, a spokesperson for operator Tokyo Electric (TEPCO), told AFP.

The reactor "is stable and there is no radioactive impact outside", he said, adding that the operator is "currently investigating the cause" of the incident and is unable to say when operations will resume.

The restart, initially scheduled for Tuesday, had been pushed back after a technical issue related to a reactor alarm was detected last weekend -- a problem that was resolved on Sunday, according to TEPCO.

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.

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

Public opinion in Niigata is deeply divided: Around 60 percent of residents oppose the restart, while 37 percent support it, according to a survey conducted in September.

"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 this week during a protest in front of the plant.

Earlier this month, seven groups opposing the restart submitted a petition signed by nearly 40,000 people to TEPCO and Japan's Nuclear Regulation Authority, saying that the plant sits on an active seismic fault zone and noted it was struck by a strong quake in 2007.