Supporters of Netiporn "Bung" Sanesangkhom put candles at a vigil in her honor late on Tuesday in Bangkok, as news spread of her death in pre-trial detention.
Thailand's Prime Minister Srettha Thavisin ordered an investigation on Wednesday into the death of the 28-year-old political activist, who died of heart failure in a prison hospital, a few weeks after partially halting a hunger strike.
"It is sad about the loss ... and I want to extend my condolences to the family," Srettha told reporters. "I have instructed the Justice Ministry to investigate details about her death."
Campaigner against laws protecting Thai royals
Netiporn was jailed in January on contempt of court charges, amid her trial for allegedly "insulting" Thailand's royal family.
The country's "lese majeste" laws are unpopular with many activists, including the Thaluwang activist group that Netiporn was part of. That group calls both for the abolition of the law protecting the royal family from supposed "insults," but also wider reforms to the monarchy.
She went on hunger strike for roughly one month in prison, but in early April began to take some soft food again.
According to Thailand's prison authorities, she had refused vitamin and mineral supplements. But they also said she was not in a critical condition before her death at a press conference on Wednesday. An autopsy was underway and results were likely soon, they said.
"Her vital signs were normal, everything was normal until the emergency," said Pongpak Areeyapinan, director of the prison hospital.
Bhumibol's death and 2020 protests renewed focus on royals
Thailand's laws on insulting the monarchy had long been a political issue, but it's intensified in the last few years.
The process probably began in 2016 with the death of the popular King Bhumibol Adulyadej at the age of 88 after a record reign of more than 70 years, having become king as a child in 1946.
His death put Bhumibol's only son, Maha Vajiralongkorn, on the throne.
The wealthiest monarch in the world, famed among other things for his residence in Germany, does not enjoy the same levels of public approval as his late father.
In 2020, widespread protests calling for reforms to the monarchy and the lese majeste laws gripped the country, led in no small part by younger activists. They also resurfaced in 2023.
Since 2020, according to the Thai Lawyers for Human Rights group, 272 people have been charged with insulting or criticizing the royal family.
The law, one of the strictest of its kind still in force anywhere in the world, can carry a maximum jail sentence of 15 years per individual violation.
msh/rc (AFP, AP, Reuters)