The 88-year-old king is the world’s longest serving monarch and is seen as a unifying force in a nation bitterly divided along political lines.

But he is hospital-bound and underwent a heart operation on Tuesday, following months of treatment for various health problems -- including water on the brain.

His health is of grave concern to Thais, who revere the king and see him as a constant in a turbulent country that has seen numerous coups and repeated rounds of deadly political violence.

Bhumibol’s image has been burnished by ritual and a publicity machine that sees giant portraits placed on streets and cinema-goers asked to stand for the royal anthem.

The leading royals are also shielded from criticism by a harsh royal defamation law that carries up to 15 years in jail for each charge.

The crown is also one of the world’s richest, with a multi-billion-dollar property and investment portfolio.

Hundreds of well-wishers gathered outside the Grand Palace in Bangkok early Thursday as a line of 770 monks began a day of celebration with an alms-giving ceremony.

“I want to make merit for the king to encourage him to have good health. That’s how I can show loyalty to him even though he can’t see it, that’s fine,” said 68-year-old Bangkok resident Chonmanee Smativat.

“I want him to know that we all love him.”

Bangkok residents of all ages donned yellow polo shirts, the king’s official colour, many emblazoned with slogans saying “We Love the King”, while banks saw queues of people eager to buy commemorative 70 baht notes.

Thais are taught from a young age about his well-publicised development schemes and economic strategies credited with lifting the kingdom out off poverty as many of its neighbours fell to communism.

But the kingdom is now cut in two with the arch-royalist Bangkok elite and south pitted against the pro-democracy north and northeast, dominated by the political dynasty of billionaire ex-premier Thaksin Shinawatra.

- AFP