BANGKOK, Jan 2 (Reuters) - Thailand's Princess Galyani Vadhana, the elder sister of revered King Bhumibol Adulyadej, has died after six months in hospital battling cancer, the palace said on Wednesday.

The king declared a 100-day mourning period and the funeral of the 84-year-old princess would be performed at Bangkok's glittering Grand Palace, the statement said.

Hundreds of Thais wearing black rushed to Bangkok's Siriraj hospital after television and radio stations reported that the princess had died before dawn.

"Although the medical team provided the best treatment, her condition deteriorated and she passed away at 0254," the statement said.

The London-born princess was diagnosed with cancer in mid-June and admitted to hospital, but her condition was not made public until October, shortly before King Bhumibol was admitted to the same hospital for restricted blood flow to the brain.

The world's longest reigning monarch was discharged three weeks later, but his illness served as a reminder to many of Thailand's 64 million people of the king's advancing years and increasing fragility.

The country laid on massive celebrations for his 80th birthday in December, just over a year after similar festivities to mark 60 years on the throne.

During his reign, Bhumibol has been a pillar of stability during often turbulent and bloody politics, including 18 military coups, the last of which was in September 2006 against Prime Minister Thaksin Shinawatra.

Despite limited constitutional powers, he wields enormous personal influence and has waded into politics on several occasions in favour of both democratic and military regimes.

Many Thais are worried for the future after his death as his heir apparent, 55-year-old Crown Prince Maha Vajiralongkorn, does not enjoy the wide respect accorded to this father.

(Reporting by Nopporn Wong-Anan, writing by Ed Cropley, editing by Darren Schuettler)

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