hailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has died after 70 years as head of state.
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.
The announcement
After two days of speculation and rumours surrounding the king's deteriorating health, the palace announcement confirming his death finally came early on Thursday evening.
"His majesty has passed away at Siriraj Hospital peacefully," a statement said, adding he had died at 15:52 (08:52 GMT).
In a televised address to the nation, Prime Minister Prayut Chan-ocha said Thailand would hold a one-year mourning period, that flags would fly half-mast and all entertainment functions must be "toned down" for a month.
Obituary: King Bhumibol
A life in pictures
King Bhumibol's reign - in brief
How King Bhumibol shaped modern Thailand
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," he said.
The king's death comes as Thailand remains under military rule following a coup in 2014.
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.
The people's grief
King Bhumibol was widely respected across Thailand, and thought of by many as semi-divine.
Hundreds of mourners gathered outside the Bangkok hospital where he died, stricken with grief at the news.
People cry after Royal Palace's announcement outside Siriraj Hospital where Thai King Bhumibol Adulyadej was being treated, in Bangkok, Thailand, Thursday, 13 October 2016.Image copyrightAP
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Crowds outside the hospital howled with grief when the announcement was made
People mourn after an announcement that Thailand's King Bhumibol Adulyadej has died, at the Siriraj hospital in Bangkok, Thailand 13 October 2016.Image copyrightREUTERS
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As night fell, crowds of mourners remained at the hospital where the King died
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.
Thai people react and mourn as they hear the news of the death of Thai King Bhumibol Adulyadej outside Siriraj Hospital in Bangkok, Thailand, 13 October 2016.Image copyrightEPA
A wave of emotion swept through the crowd - Jonah Fisher, BBC News, Bangkok
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 was 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 local time, 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.
In pictures: Thailand mourns King Bhumibol
The succession
Although the prime minister said Crown Prince Maha Vajiralongkorn would become the new monarch, he added that the official proclamation would be made at a later date.
He said the crown prince had confirmed that he would perform his duty as heir to the throne, but had asked for time to mourn his father's death.
This file photo taken on 11 May 2009 shows Thai Crown Prince Maha Vajiralongkorn observing the annual ploughing ceremony at Sanam Luang in Bangkok.Image copyrightAFP
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Most ordinary Thais know only a few details about Crown Prince Vajiralongkorn
The crown prince, who is 64, 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.
Profile: Crown Prince Vajiralongkorn
Strict lese-majeste laws protect the most senior members of Thailand's royal family from insult or threat. Public discussion of the succession can be punishable by lengthy jail terms.
Given the pivotal role the king has played in maintaining the balance of power in Thailand's volatile political environment, the succession will be a formidable challenge for the government, says the BBC's Jonathan Head in Bangkok.
King Bhumibol's family tree
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The r