Thailand is marking 70 years since King Bhumibol Adulyadej, the world's longest-reigning living monarch, ascended to the throne.
The celebrations began with a religious ceremony in Bangkok, led by 770 Buddhist monks, an auspicious number.
The 88-year-old king is revered by Thais, for whom he has been a figure of stability through the country's decades of political upheaval.
But he is in poor health and has not been seen in public for months.
On Tuesday, he had heart surgery, with what the palace said were "satisfactory results".
The procedure, known as balloon surgery, was to widen his arteries after tests showed he had insufficient blood in the heart muscles, the statement said.
The king has spent much of the past few years in hospital, and his health is closely watched by Thais.
"The relationship between Thais and the King is deep, more than one can actually begin to explain," Col Winthai Suvaree, a spokesman for the royalist junta, said to Reuters.
"He is a father to the land."
Several hundred people had gathered outside the palace on Thursday morning to wish the king well.
He is not expected to make a public appearance.
"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," 68-year-old Bangkok resident Chonmanee Smativat told AFP.
"I want him to know that we all love him. He has worked hard for Thais. Since I was little I have seen how hard he works."
The king's popularity stems partly from his long reign and service to Thai people, but also because he is seen as an arbiter in politics.
Thailand has been wracked by political strife in recent years and is currently governed by a military-led government.
Strict lese majeste laws ban any criticism of the king or the royal family.
