King Bhumibol was widely respected across Thailand, and thought of by many as semi-divine.
Hundreds of mourners have gathered outside the Bangkok hospital where he died, stricken with grief at the news.
Many held pictures of the king and wore yellow or pink clothing in his honour.
"How will Thailand live without you father?" cried one distressed man.
"I lost one of the most important people in my life. I feel like I haven't done enough for him," said another.