Thailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has died after 70 years as head of state, the palace says.
The 88-year-old king was widely revered but had been in poor health in recent years, making few public appearances.
He was seen as a stabilising figure in a country hit by cycles of political turmoil and multiple coups.
Crown Prince Maha Vajiralongkorn will be the new monarch, the prime minister has said.
In a televised address to the nation, Prayut Chan-ocha said Thailand would hold a one-year mourning period and that all entertainment functions must be "toned down" for a month.
"He is now in heaven and may be looking over Thai citizens from there," he said of King Bhumibol.
The palace had warned on Sunday that the king's health was "not stable".
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, many stricken with grief at the news.
