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.

ink has been associated with King Bhumibol Adulyadej for many years, after royal astrologers suggested the monarch wear the color to stimulate good health and well-being. Over the years, the King has often attended public occasions dressed in pink blazers.

Thais nationwide are also dressing in yellow to express their respect for the King`s good health. In Thai culture, colors are assigned to each day of the week. King Bhumibol Adulyadej was born on a Monday, and so Thais have traditionally donned the day`s corresponding color of yellow to honor him.

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.

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