Thailand on Thursday marked the 70th anniversary of King Bhumibol Adulyadej's accession to the throne.
Roads around the Grand Palace in Bangkok were closed for an alms-giving ceremony involving 770 Buddhist monks to celebrate the king's reign.
Hundreds of Thais gathered before dawn to take part in the ceremony which was led by Prime Minister Prayut Chan-ocha and members of the cabinet. Almost all wore yellow, the official colour of the king.
Bhumibol, 88, the world's longest reigning monarch, has spent much of the past several years in hospital battling various ailments.
"He is the only king I've ever known. I pray for his speedy recovery so that he can continue to guide us, his children," said Usa Paeliew, a farmer from nearby Pathum Thani Province.
"If I live a hundred lifetimes, I would consider myself lucky if I were living under his majesty's bloodline," said Somporn Maejai, a sanitary worker from the capital.
Heads of states from around the world also sent messages of congratulations.
Queen Elizabeth II of the United Kingdom offered her "warmest congratulations" noting that "the relationship between our two countries ... has become even stronger during Your Majesty's reign."
President Barack Obama, who met with Bhumibol in 2012, also released a statement expressing his "admiration."
"His Majesty’s leadership has helped our two countries promote peace and prosperity throughout the Asia-Pacific region," the statement said.
Royal doctors said Tuesday that Bhumibol had undergone a heart operation to expand an artery. He was last seen in public in January.
The king is viewed as a unifying figure in Thailand. Anxiety over his health and the royal succession has added to the political uncertainty surrounding military-ruled Thailand.