By Mari Saito and Tetsushi Kajimoto
TOKYO (Reuters) - In a rare move by a former Japanese prime minister, Yukio Hatoyama joined a boisterous anti-nuclear demonstration outside his old office on Friday, a fresh sign that the ruling party he once led is fracturing over energy and other policies.
Japan's debate over nuclear power has become increasingly heated after incumbent Prime Minister Yoshihiko Noda's decision to restart idled reactors despite persistent public safety concerns following last year's Fukushima nuclear crisis.
The question of nuclear power's role in a new energy portfolio the government is set to decide next month is adding to divisions in Noda's Democratic Party of Japan (DPJ), already rent by feuds over his plan to double the sales tax to curb debt and the possibility Tokyo might join a U.S.-led trade deal.
On Monday, an estimated 100,000 anti-nuclear protesters took to the streets in Tokyo, while ever-bigger crowds have been gathering every Friday outside Noda's office.
"It is truly regrettable that the voices of all of you gathered here today are so far removed from politics and the prime minister's office," said Hatoyama, wearing a clear raincoat under a steady drizzle and surrounded by reporters.
"As a former prime minister ... I want to take your message inside the prime minister's office," he said after shaking hands with a few of the thousands of demonstrators.
He then entered Japan's equivalent of the White House, where Kyodo news agency said he met Chief Cabinet Secretary Osamu Fujimura.
Hatoyama took office in 2009 when the Democrats swept to power for the first time but quit after less than a year in office, felled by charges of incompetence and failure to keep a campaign promise to move a U.S. military base off the southern island of Okinawa.
Hatoyama's participation was cheered by some protesters but dismissed by others as grandstanding.
"He can come here and say something impressive but it doesn't really matter," said Osamu Arai, a 65-year-old construction worker taking part in the demonstration.
"This is a grass roots movement. Things change very slowly in Japan, but we must continue to protest."
Hatoyama's gesture however was another sign that Noda's Democratic Party is in danger of unraveling further.
Earlier this week, three members of parliament's upper house left the DPJ, citing opposition to the reactor restarts, the sales tax and the possibility that Japan might join a U.S.-led free trade pact.
They were the latest to bolt after former DPJ leader Ichiro Ozawa, an unpopular political veteran, led dozens out of the DPJ to set up a new party, also protesting against the sales tax and promising to wean Japan from its reliance on nuclear power.
Hatoyama, who also opposes the tax hike, has already hinted that he and his DPJ backers might follow suit.
The government is set to decide on a new energy mix next month to replace a scrapped 2010 program that had sought to raise nuclear power's share to more than half of electricity needs by 2030 from about 30 percent before the Fukushima crisis triggered by the huge earthquake and tsunami in March 2011.
Most experts expect Noda to opt for a 15 percent share for atomic power in electricity supply by 2030, an option that would require the restart of all 50 of Japan's reactors -- all but two of which are now idled for safety checks -- before gradually closing older units.
Two reactors in western Japan have resumed operations, one just this week.
Noda has been hit by a string of setbacks on nuclear policy this week.
First, critics blasted the government's handling of public hearings on the energy portfolio after employees of utilities were among the few chosen to speak.
Then on Friday, media leaks forced the government to delay nominating candidates for a new atomic safety watchdog that it hopes will instil more confidence than current regulators, criticized for their cosy ties with the power industry.
Newspapers reported on Friday that the government had picked Shunichi Tanaka, 67, an expert in radiation physics and a former deputy head of the Cabinet Office's Atomic Energy Commission, to head the new safety regulator.
Tanaka drew mixed reviews from anti-nuclear groups, with some saying he represented Japan's "nuclear village" -- a nexus of politicians, utilities and regulators that experts say was a major factor in the failure to avert the Fukushima disaster.
Others, though, noted that after the disaster Tanaka spoke of the need to reflect on the way nuclear power had been promoted.
"We can be somewhat positive, but we cannot have great hopes," said Hideyuki Ban, secretary-general of the Citizens' Nuclear Information Center.
(Editing by Ed Lane)