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."

FURTHER FRACTURES

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. Continued...