By Kentaro Hamada

NIIGATA, Japan (Reuters) - Shares in Tokyo Electric Power <9501.T> fell 8 percent on Monday after an anti-nuclear candidate won an upset victory in a Japanese regional election, in a blow to its attempts to restart the world's biggest atomic power station and a challenge to the government's energy policy.

The election of Ryuichi Yoneyama, 49, a doctor-lawyer who has never held office, is a setback for Prime Minister Shinzo Abe's energy policy, which relies on rebooting reactors that once met about 30 percent of the nation's needs. All but two are shut down in the wake of the 2011 Fukushima nuclear disaster.

Reviving the seven-reactor giant, with capacity of 8 gigawatts, is key to saving Tepco, which was brought low by the Fukushima explosions and meltdowns, and then the repeated admissions of cover-ups and safety lapses after the world's worst nuclear disaster since Chernobyl in 1986.

Yoneyama won the vote on Sunday after a campaign dominated by concerns over the future of the Kashiwazaki-Kariwa power station and nuclear safety, beating Tamio Mori, 67, who was backed by the ruling Liberal Democratic Party (LDP) and initially favored for an easy victory.

"As I have promised all of you, under current circumstances where we can't protect your lives and your way of life, I declare clearly that I can't approve a restart," Yoneyama told supporters at his campaign headquarters.

Tepco shares were down by 7.4 percent at 385 yen at 0100 GMT (09:00 p.m. EDT) after falling further earlier. The Nikkei 225 was up by 0.5 percent and other utilities were mixed.

Yoneyama had more than 500,000 votes to about 430,000 for Mori with 93 percent of the vote counted in the region on the Japan Sea coast, public broadcaster NHK said.

Mori, a former construction ministry bureaucrat, apologized to his supporters for failing to win the election.

Yoneyama, who had run unsuccessfully for office four times, promised to continue the policy of the outgoing governor who had long thwarted the ambitions of Tepco, as the company supplying about a third of Japan's electricity is known, to restart the plant.

As the race tightened, the election became a litmus test for nuclear safety and put Abe's energy policy and Tepco's handling of Fukushima back under the spotlight.

"The talk was of Kashiwazaki-Kariwa, but I think the result will affect nuclear restarts across the country," said Shigeaki Koga, a former trade and industry ministry official turned critic of nuclear restarts and the Abe administration.

Koga told Reuters it was important that Yoneyama join forces with another newly elected governor skeptical of nuclear restarts, Satoshi Mitazono of Kagoshima Prefecture in southern Japan. "Without strong support from others, it won't be easy to take on Tepco," he said.

TROUBLES

Tepco spokesman Tatsuhiro Yamagishi said the company couldn't comment on the choice of Niigata governor but respected the vote and would strive to apply the lessons of the Fukushima disaster to its management of Kashiwazaki-Kariwa.

The government wants to restart units that pass safety checks, also promoting renewables and burning more coal and natural gas.

Only two of Japan's 42 reactors are running more than five years after Fukushima, but the Niigata plant's troubles go back further.

Several reactors at Kashiwazaki-Kariwa have been out of action since an earthquake in 2007 caused radiation leaks and fires in a disaster that prefigured the Fukushima calamity and Tepco's bungled response.

Niigata voters opposed restarting the plant by 73 percent to 27 percent, according to an NHK exit poll.

Yoneyama, who has worked as a radiological researcher, said on the campaign trail that Tepco didn't have the means to prevent Niigata children from getting thyroid cancer in a nuclear accident, as he said had happened in Fukushima. He said the company didn't have a solid evacuation plan.

The LDP's Mori, meanwhile, was forced to tone down his support for restarting the plant as the race tightened, media said, insisting safety was the top priority for Kashiwazaki-Kariwa, while promoting the use of natural gas and solar power in Niigata.

(Reporting by Kentaro Hamada; Writing by Aaron Sheldrick; Editing by William Mallard and Ros Russell)