The palace said the king passed away peacefully on Thursday at Bangkok's Siriraj Hospital.
The longest-reigning monarch of the world at the time of his passing, King Bhumibol has overseen his people through a lifetime, nurturing and bringing stability to the country through some of the most decisive moments of the past decades.
The king's heir apparent is his son, Crown Prince Vajiralongkorn, though there are other possible candidates for succession.
Prime Minister Prayut Chan-o-Cha confirmed the succession in a statement broadcast on all Thai television channels.
Adulyadej enjoyed traveling through croplands in distant provinces to tend to the many development projects he managed - projects including his "milk-pasteurizing plants, dams that watered rice fields, factories that recycled sugar-cane stalks and water hyacinths into fuel, and countless others", the Times said. We need to do everything smoothly. "He was revered by the people of Thailand and highly respected internationally".
The 64-year-old prince has been Bhumibol's named successor since 1972.
On Sunday, the palace announced his health had become "unstable" and yesterday Vajiralongkorn rushed back from Germany, Mr Chan-ocha canceled a trip overseas and royal family members began gathering at Siriraj hospital.
"The king's great contribution in deepening friendship between Japan and Thailand will be remembered by all Japanese people", he said.
Many among the crowd prayed, sang the royal anthem and repeatedly shouted: "Long live the King". I can't hold my tears.
"Furthermore, people should use this opportunity to support one another because all of us may be different but we have one heart because we have a shared father of the land".
"He is a father ... that wanted to do everything, the best thing, for his kids", she said.
"I hope everyone will understand and not cause chaos", he added.
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"I still believe there can be a miracle".
The government announced a 100-day mourning period and a 30-day moratorium on state events. The royal palace said in a statement late Wednesday that the 88-year-old kin.
European leaders are expressing sympathy following the death of Thailand's king on Thursday.
"People have anxieties and apprehensions about the near future because the king's role in the past has been so indispensable and instrumental in seeing Thailand climb from being a backwater village to a modern nation", said Thitinan Pongsudhirak, a politics expert at Chulalongkorn University.
Pimook came to the hospital with his girlfriend Aunchisa Saekuay, who said the restaurant she runs was closed until further notice.
India's Prime Minister Narendra Modi tweeted a condolence message after the death Thursday of Thailand's king.
The king's appeal stretched from the rich urban elite to the poor agricultural workers in the provinces.
But in Thailand there is often a gap, sometimes of many years, between assuming the throne and being crowned. Until his death, he reigned over more than 20 prime ministers and repeated constitutional changes.
"The Americans went all out with a mix of soft entertainment and gushing political recognition that put Bhumibol in a light comparable to that of the West's most powerful leaders", Paul Handley wrote in a biography of Bhumibol, "The King Never Smiles". He was the ninth king in the Chakri Dynasty, which dates back to 1782.
As his health deteriorated, the king's participation in public affairs sharply declined in recent years.
However, the Royal Palace said Adulyadej died in a "peaceful manner".
Toward the end of his reign, his presence in public life waned. He had surgery to remove his gallbladder previous year. His birthday is celebrated by all and his pictures are everywhere.