- Doctors said King Bhumibol Adulyadej's condition was "unstable"
- He underwent a series of medical procedures over the weekend
The 88-year-old, who ascended to the throne 70 years ago -- making him the world's longest-reigning monarch -- is being closely monitored after undergoing medical procedures over the weekend.
Thais wearing pink shirts are saying prayers at their local temples and visiting the Siriraj hospital in Bangkok, where the King is being treated, according to images on social media.
Messages of support have been flooding onto social media sites including Facebook.
Doctors at the Siriraj hospital in Bangkok said
that the King was having problems with his kidneys, blood flow to the left side of his heart and blood pressure.
He was also put on a ventilator and his general condition "remains unstable," according to the statement from the Bureau of the Royal Household which was posted on the King's official Facebook page on Sunday.
Concerns for the highly revered King have been mounting in recent years due to his extended hospital stays.
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.