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.
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.
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.