Thailand's former Queen Sirikit, the mother of the current King Vajiralongkorn and wife of the nation's longest-reigning monarch, died late on Friday at the age of 93, the palace said.
"It is a great loss to the nation," said Prime Minister Anutin Charnvirakul on Saturday, adding that he would delay his departure to a weekend summit of Southeast Asian leaders in Malaysia.
Throughout her 66-year marriage to King Bhumibol Adulyadej, Sirikit carved a dual reputation as a fashionista and the nation's caring matriarch - with some Western media featuring her on magazine covers and comparing her to former US first lady Jackie Kennedy.
"I had heard that she had been unwell and, given that she was in her 90s, I knew this day would come," 53-year-old housekeeper Sasis Putthasit said early on Saturday in the capital Bangkok.
"But I didn't expect it to be today," Sasis told reporters. "I feel sad because she was a mother figure to the country, and now she's gone."
Sirikit had "suffered several illnesses" while hospitalised since 2019, including a blood infection this month, the palace said in a statement.
"Her majesty's condition worsened until Friday and she passed away... at Chulalongkorn hospital at age 93," it added.
King Vajiralongkorn has assigned members of the royal family to begin a year-long mourning period, the palace said.
From early Saturday, Thai news anchors were seen wearing black during broadcasts, a sign of public mourning.
Her passing "marks a profoundly significant event for the Thai Royal Family and the entire nation, given her immense popularity and deep connection to the late king who remains deeply revered," Pavin Chachavalpongpun, a former Thai diplomat and academic who studies the monarchy, told reporters.
King Bhumibol Adulyadej's lengthy reign from 1946 until 2016 was bookended by World War II and the first inauguration of US President Donald Trump.
Though Bhumibol's son inherited the throne about nine years ago, many still revere him as the nation's most steadfast figurehead - and Sirikit as his constant companion.
She retired from the public eye in recent years as she suffered from ailing health, her privacy sealed by strict lese majeste laws that limit what can be reported about the royal family.
But in her glamorous heyday in the 1960s she mingled with US presidents and superstars such as Elvis Presley, while at home touring Thailand to visit villagers in rural areas.
She was referred to as the "Mother of the Nation" and her birthday was designated the country's Mothers' Day.
Store owner Tanyaporn Arammetha, whose own parents divorced, said she always saw the late king and queen as parental figures.
Agence France-Presse