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.
Describing the king’s death as “the most devastating moment for Thais”, the prime minister said: “He is now in heaven and may be looking over Thai citizens from there.”
In a later statement he urged vigilance, saying national security was a top priority.
“Everyone will need to be alert in every region and throughout the country to ensure safety”, the Reuters agency reported him as saying.
The king’s death comes as Thailand remains under military rule following a coup in 2014.
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.
A palace official, speaking to crowds outside the hospital, said the body of the king would be moved to the Temple of the Emerald Buddha in Bangkok on Friday.
“His majesty has passed away at Siriraj Hospital peacefully,” the palace statement said earlier, adding he had died at 15:52 (08:52 GMT).
Throughout the first part of the day the king’s supporters had sung royal songs and recited prayers at Siriraj Hospital. Many wore carefully chosen colours. Pink, for good health, or yellow the royal colour.
Most knelt holding pictures of King Bhumibol, facing the part of the hospital where he had been receiving treatment.
By early afternoon the mood had begun to change. With all of his children at his bedside, rumours were circulating that the 88-year-old had passed away.
The tears began to flow, and in between songs the call of “God Save the King” was shouted with an almost visceral desperation.
Then just before 19:00 the news was official. A wave of emotion swept through the crowd. Many clung to each other and wept.
Thailand’s father figure, a beacon of stability in a divided and worried country, had died. A new, more uncertain era has begun.