A Japanese royal - who served World War Two and criticised the brutality of troops fighting in his older brother's name - has been laid to rest after his death at the age of 100.

Prince Mikasa, the youngest brother of Japan's wartime ruler Emperor Hirohito, was the uncle of current monarch Emperor Akihito.

Today solemn ceremonies were held in memory of a man who challenged the notion that his brother was divine, and called for his abdication after Japan's war defeat.

Japan's Crown Prince Naruhito, second from left, Crown Princess Masako, left, and other royal family attend the funeral service for Prince Mikasa

The royal, pictured with Princess Yuriko in 1941 (left) and last year (right) was 100 when he died

A car carrying the casket of the late Prince Mikasa, uncle of the current Emperor Akihito, arrives at the funeral

Prince Mikasa, the oldest monarch in Japanese history, was famously appalled by the atrocities he witnessed, and vocally criticised Japan's aggression during the war.

Although the conflict was presented to citizens as a sacred war, the royal came to question this. He wrote a scathing report about the brutality he witnessed, revealing chemical warfare experiments had been carried out on prisoners of war in Manchuria, and claimed this was fuelling Chinese resistance.

The document was kept secret by military chiefs, and the public only became aware of it 50 years later.

Akihito's heir, Crown Prince Naruhito and his wife, Crown Princess Masako, were in attendance along with dignitaries including Prime Minister Shinzo Abe and US ambassador to Japan, Caroline Kennedy were among those who attended the funeral

Japan's Princess Mako (right) and Princess Kako (left) attend the funeral of Prince Mikasa, at the Toshimagaoka cemetery in Tokyo

Mikasa, the fourth son of the Emperor Taisho and Empress Teimei, also alleged that an attempt had been made to poison a League of Nations delegation which was investigating a bombing by Tokyo.

In a 1994 interview with the Yomiuri newspaper, Mikasa said: '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.'

Following the war, Mikasa called for his brother to abdicate, supported by imperial princes at a privy council meeting.

Japan's Princess Yuriko, wife of late Prince Mikasa, is seen after praying at the altar during his funeral

Japan's Prime Minister Shinzo Abe bows before praying at the altar during the funeral service

He said his brother should take responsibility for what had happened. But Hirohito refused, and continued to rule until his death in 1989.

Mikasa considered renouncing his own title, but opted against such an extreme measure.

A critic of the pomp and ceremony that surrounded the royal court, he travelled by train to the Japanese Society for Middle Eastern Studies, which he founded, and ate his lunch around the students.

Japan's Crown Prince Naruhito (centre) and Crown Princess Masako bow during the funeral service for late Prince Mikasa

He preferred to be addressed as 'Mikasa-san' like ordinary Japanese people.

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.

The late prince and wife Princess Yuriko pictured during a Canadian tour

The prince was a fierce critic of Japan's brutality during World War Two, and called for his brother to abdicate following the country's defeat

Akihito's heir, Crown Prince Naruhito and his wife, Crown Princess Masako, were in attendance along with dignitaries including Prime Minister Shinzo Abe and US 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.

Japan's Crown Prince Naruhito, third from left, Crown Princess Masako, second from left, Prince Akishino, left, and other royal family bow during the funeral service

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.