King Bhumibol Adulyadej is much revered and thousands gathered to hear him speak at his palace in the coastal resort of Hua Hin.
He called on people to do their "duty" to support each other for the sake of the country and said Thailand had been peaceful because of the unity of the people.
"All Thais should realise this point a lot and behave and perform our duties accordingly, our duty for the sake of the public, for stability, security for our nation of Thailand," the King said.
Anti-government protesters are demanding that the current government resign and began marching on the streets of Bangkok on 24 November.
They agreed to stop their attacks on government buildings for the birthday celebrations.
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