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.
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."
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.
"His majesty has passed away at Siriraj Hospital peacefully," the palace statement said earlier, adding he had died at 15:52 (08:52 GMT).
Parliament is to hold a special session at 21:00 local time (14:00 GMT).
The king's death comes as Thailand remains under military rule following a coup in 2014.
Crown Prince Vajiralongkorn, who is 63, is much less well known to Thais and has not attained his father's widespread popularity. He spends much of his time overseas, especially in Germany.
Tributes have been flowing in from across the world.
US President Barack Obama called him a "tireless champion of his country's development".
"I had the honour of calling on his majesty the king during my visit to Thailand in 2012, and recall his grace and warmth, as well as his deep affection and compassion for the Thai people," he said.
UN Secretary General Ban Ki-Moon said in a statement that he hoped Thailand would honour King Bhumibol's legacy of commitment to universal values and respect for human rights.
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