The celebrations began on Thursday with a religious ceremony in Bangkok, led by 770 Buddhist monks. This is seen as an auspicious number.

Hundreds of well-wishers, many dressed in the royal color yellow, gathered outside the Grand Palace for an alms-giving ceremony.

Ill health and uncertainty over the future

The monarch - one of the world's richest, with a multi-billion-dollar property and investment portfolio - has spent much of his time over the past few years in hospital due to a variety of health problems, adding to political uncertainty surrounding military-ruled Thailand.

He had a heart operation on Tuesday, following months of treatment for various health problems, including water on the brain.

A uniting presence

The elderly king is widely seen as a unifying factor in a country split between a Bangkok elite and the pro-democracy north and northeast, dominated by the political dynasty of billionaire ex-premier Thaksin Shinawatra.

Stringent royal insult laws prevent criticism of the king and have been enforced with record jail terms under a junta that seized power in 2014.

Questioning and criticism of the monarchy is banned and there has been a surge of lese majeste convictions, some up to 30 years, since the army grabbed power in 2014.

Crown Prince Vajiralongkorn, the king's 63-year-old son, is the heir apparent. He has not achieved the same level of devotion that his father enjoys.

An inauspicious beginning

The second son of Prince Mahidol and the grandson of King Chulalongkorn, Bhumibol was not initially groomed for the role of monarch.

Bhumibol ascended to the throne on June 9, 1946,after his brother was found dead in a Bangkok palace from a gunshot wound, a death the king described decades later in a BBC documentary as "very mysterious."

The 18-year-old prince - who had spent most his life abroad in Switzerland and the United States - inherited a throne that was widely seen as being on the edge of obsolescence, some 14 years after losing absolute power in a coup d'etat in the capital Bangkok.

Revitalizing the monarchy

Under military stewardship, Bhumibol and his wife, Sirikit, toured Thailand and abroad in a public relations bid to revitalize the monarchy and the country.

jbh/sms (dpa, AFP, Reuters)