Tokyo Electric Power Company (TEPCO) said it would need another day of two to check the equipment at the Kashiwazaki-Kariwa plant, which media reports said was set to restart on Tuesday.

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

The Kashiwazaki-Kariwa facility would be the first nuclear plant that Fukushima operator TEPCO restarts since the disaster.

The company has never publicly announced a date to switch on the plant.

TEPCO has decided to run more checks after detecting a technical issue on Saturday related to an alarm linked to one of the reactors at Kashiwazaki-Kariwa, company spokesman Isao Ito told AFP.

The alarm issue had been fixed by Sunday, he added.

After the final checks, the utility will explain to nuclear authorities what had happened and proceed to restart the plant, the spokesman said, without providing an exact timeline.

More than a decade since the Fukushima accident, Japan now wants to revive atomic energy to reduce its dependence on fossil fuels, achieve carbon neutrality by 2050 and meet growing energy needs from artificial intelligence.

But it is a divisive issue, with many residents worried about nuclear safety.

About 50 people gathered Monday outside TEPCO's headquarters in the capital Tokyo, chanting "No to the restart of Kashiwazaki-Kariwa!"

"TEPCO only mentions a possible delay. But that's not enough," said Takeshi Sakagami, president of the Citizens' Nuclear Regulatory Watchdog Group.

"A full investigation is needed, and if a major flaw is confirmed, the reactor should be permanently shut down," he said at the rally.

The reactor has cleared the nation's nuclear safety standard.

Prime Minister Sanae Takaichi has voiced her support for the use of nuclear power.

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