Thailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has died after 70 years as head of state, the Thai 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, Thai Prime Minister Prayut Chan-ocha said.
In a televised address to the nation, Mr Prayut 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", Mr Prayut said: "He is now in heaven and may be looking over Thai citizens from there."
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," another said.
Crowds outside the hospital howled with grief when the announcement was made
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 3:52pm (9.52pm NZT).
Parliament was due to hold a special session at 9pm local time.
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.
Strict lese-majeste laws mean public discussion of the succession are 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.
King Bhumibol, who was born in Cambridge in the US state of Massachusetts, ascended to the throne on 9 June 1946 after his brother, King Ananda Mahidol, died.
Though a constitutional monarch with limited powers, many Thais looked to King Bhumibol to him to intervene in times of high tension.
He was seen as a unifying and calming influence through numerous coups and 20 constitutions.
However, his critics argued he had endorsed military takeovers and at times had failed to speak out against human rights abuses.
Following the death of King Bhumibol, Britain's Queen Elizabeth II has become the world's longest-reigning monarch, having been on the throne for 64 years.