Ovartvoraporn Bhakchuda, a Thai woman who spoke to CNN outside the hospital, was unable to hold back her tears, saying that she hoped the news was not true.
"We lost our father today," she said.
"He is a father ... that wanted to do everything, the best thing, for his kids," she said.
"With all my heart I hope that miracle will happen. I want to believe in miracles...I still believe up until now that this is a lie."
According to the palace statement, all government buildings will fly the Thai flag at half-staff for 30 days starting October 14. And in a sign of how deeply this leader's death will be marked, all civil servants have been ordered to wear black clothing for a year as a sign of mourning, the palace said.
Tributes from around the world
Condolences began pouring in from world leaders. US President Barack Obama said he recalled the king as a man of "grace and warmth," and his "deep affection and compassion for the Thai people," when he visited the monarch in 2012.
"As the revered leader and only monarch that most Thais have ever known, His Majesty was a tireless champion of his country's development and demonstrated unflagging devotion to improving the standard of living of the Thai people," Obama said.
British Prime Minister Theresa May said: "His Majesty guided the Kingdom of Thailand with dignity, dedication and vision throughout his life. He will be greatly missed."
Indian Prime Minister Narendra Modi described the king as "one of the tallest leaders of our times."
A 70-year reign of 'respect'
The king's appeal stretched from the rich urban elite to the poor agricultural workers in the provinces. During his reign, he skilfully charted a course that put the monarchy at the center of Thai society, acting as a force for community and tradition even as the country lurched between political crises and more than a dozen military coups, both attempted and successful.
He acceded to the throne in 1946 in the aftermath of Thailand's occupation by Japan during the Second World War, and ended deep into the Internet age. Until his death, he reigned over more than 20 prime ministers and repeated constitutional changes. He also helped the country navigate the disruptive effects of the nearby Vietnam War during the 1960s and '70s.
For most Thais, he is the only monarch they have ever known, and before his death, analysts expressed concern that his passing will remove a vital point of unity in an increasingly divided country.
Also known as Rama IX -- a reference to his lineage stretching from Rama I, the founder of the Chakri dynasty -- Bhumibol commanded great love and respect within Thailand. An energetic public relations machine promoted his popularity, which led to his portrait being adorned with marigolds from the marble halls of Bangkok office lobbies to the poorest of rural homes.
The king's health had been in slow decline for some time. The palace said that a team of royal physicians had tried their best and provided the closest care.
Toward the end of his reign, his presence in public life waned. Doctors at the Siriraj Hospital had earlier said that king was having problems with his kidneys, blood flow to the left side of his heart and blood pressure.
Last month, he was treated for a severe infection, reduced kidney function and fluid in his lung. He had surgery to remove his gallbladder last year.