King Bhumibol Adulyadej, 88, is the world's longest-reigning monarch and beloved by many in Thailand.
But he has not been seen in public for nearly a year as he battles a series of ailments in a Bangkok hospital.
Fears over his health have grown this week after the palace announced on Sunday that his condition was "not stable", with doctors recommending he suspend all royal duties.
AFP reporters outside the hospital said extra police were drafted in on Wednesday, some of whom joined well-wishers in prayers.
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A hospital spokeswoman said the king's nominated successor Crown Prince Maha Vajiralongkorn, who spends much of his time abroad, was planning to visit.
"The security has been stepped up because the Crown Prince is coming to the hospital," she said, asking not to be named.
Thai media travelling with Prime Minister Prayut Chan-O-Cha in the eastern province of Chonburi said the junta chief had cancelled all official engagements Wednesday afternoon and returned to the capital.
No reason was given, according to media which reported the cancellation including the major dailies Matichon and Thai Rath.
Bhumibol's health is a taboo subject and palace officials maintain tight control on news about his condition.
A draconian lese majeste law also makes public discussion of the succession all but impossible.
Sunday's statement was unusually grim in its prognosis. Previous statements have tended to end on a positive note after successful treatment.
The king has battled a range of ailments in the last two years including regular infections, breathing difficulties, renal failure and hydrocephalus -- a build-up of cerebrospinal fluid commonly referred to as "water on the brain".
The announcement caused significant business jitters this week.
The main bourse plunged after the lunch break on Wednesday, dropping as much as 6.8 percent. On Monday it declined 3.15 percent and on Tuesday it fell 1.02 percent.
Most Thais have known no other monarch and Bhumibol is widely seen as a unifying symbol in a country rocked by decades of political turbulence and divisions.
Privately many business leaders -- both domestic and foreign -- fret that his demise could lead to economic instability, especially as there is no official discussion on how the country will handle his passing.
