Many in the crowd were tearful as the royal motorcade transported King Bhumibol Adulyadej from the hospital where he has spent the last three years to the Ananta Samakhom Throne Hall, the Bangkok Post reported.
The king's birthday speech, delivered from the balcony of the throne hall, was broadcast live in Thailand and watched by millions of people across the country.
"The goodwill and harmony from you that I have witnessed today makes me feel joy and serves as my encouragement," the king said. "The blessings and vows that you have uttered are very impressive."
The Thai king, who came to the throne is 1946, is the world's longest reigning monarch, the official Chinese news agency Xinhua reported.
He was admitted to the hospital in September 2009 for pneumonia and has remained there, rarely making public appearances.
A statement issued by the Royal Household Bureau said Queen Sirkit did not accompany her husband to the ceremony on the advice of her doctors.