Parichart Kaewsin, 35, who works in a bank, stood at the edge of the Siriraj hospital garden, gazing up at the top floor of the building where the king was treated.
"I knew he was sick but I still can't believe this day has come," she said, choking back tears. "That's why I came here - to hear for myself. He was like our dad."
Al Jazeera's Scott Heidler, reporting from the capital Bangkok, said the government announced a mourning period of one year.
"He had been a fabric of Thai life for the last 70 years that he has reigned as king. He has been partially lifted up to a god-like status here, because of what he has done for this country," said Heidler.
Born on December 5, 1927 in the United States, Bhumibol came to the throne aged 18 in 1946 when his brother was shot dead in mysterious circumstances at the Royal Palace in Bangkok.
He was crowned four years later after finishing his studies in Switzerland.