By Elaine Lies

TOKYO (Reuters) - Japanese Emperor Akihito's uncle, Prince Mikasa, who served in China during World War Two and criticized the war waged in his older brother's name, was laid to rest on Friday in solemn ceremonies attended by royals, the premier and other mourners.

Mikasa's death at the age of 100 - the oldest Japanese royal in recorded history - leaves just four heirs to the Chrysanthemum throne.

His death comes amid renewed attention to the future of a monarchy whose past traditionalists say stretches back 2,600 years and whose future currently rests with one 10-year-old boy. Women cannot ascend to the throne.

A Shinto priest in white robes walked slowly ahead of the hearse at Tokyo's Toshimagaoka cemetery under bright blue skies to the sound of "shakuhachi" flute music. Mikasa's 93-year-old widow, Princess Yuriko, followed in a wheelchair.

Akihito's heir, Crown Prince Naruhito and his wife, Crown Princess Masako, were in attendance along with dignitaries including Prime Minister Shinzo Abe and U.S. ambassador to Japan Caroline Kennedy. In line with tradition, Akihito and Empress Michiko did not attend.

After a reading by a priest, chief mourners laid offerings of ritual greenery at an alter after which others approached and bowed to pay their respects.

The youngest brother of Emperor Hirohito, who until Japan's defeat was worshipped as a god, Mikasa served in the military and was posted to Nanjing for about a year from 1943.

China says Japanese troops killed 300,000 people in 1937 in its then capital of Nanjing. A postwar Allied tribunal put the death toll at 142,000, but some conservative Japanese politicians and scholars deny a massacre took place at all.

In a 1994 interview with the Yomiuri newspaper, Mikasa was quoted saying "I was really shocked when an officer told me that the best way to train new soldiers was to use living prisoners of war for bayonet practice because it gave them will power."

An Oriental History scholar, Mikasa eschewed royal honorifics, preferring to be addressed "Mikasa-san" like ordinary Japanese. He was also a folk dancing aficionado and enthusiastic ice skater, and enjoyed karaoke.

Akihito, 82, hinted in August that he wanted to abdicate - a step unprecedented in modern Japan and not possible under current law.

The remaining four male heirs include 10-year-old Prince Hisahito, the emperor's only grandson, raising concerns about the monarchy's future unless reforms to allow women to inherit and pass on the throne are enacted.

"I hope the passing of Prince Mikasa will become an opportunity to think a bit more about all these issues regarding the imperial family and succession," said Naotaka Kimizuka, a specialist in European monarchies at Kanto Gakuin University.

The three older heirs are Akihito's 80-year-old brother and his two middle-aged sons including Naruhito.

Mikasa's body will be cremated and interred at the cemetery later in the day, public broadcaster NHK said.

(Additional reporting by Kwiyeon Ha; Additional reporting and writing by Linda Sieg)