The length of the king's hospital stay and the lack of detailed information from the palace have created widespread worry about his condition.
Bhumibol, the world's longest-reigning monarch, is widely revered in Thailand and his health is an extremely sensitive topic, mainly because of concerns about the royal succession.
The king's son, Crown Prince Vajiralongkorn, does not yet have the stature or moral authority of his father, who has acted as a unifying figure for the 63 years he has been head of Thai kingdom.
"I have been praying every night asking all holy spirits in the universe to help with his quick recovery"
With concerns growing over the king's health, thousands of Thais have gathered daily at Siriraj Hospital where Bhumibol is being treated to sign get-well books or lay garlands in front of a monument of Prince Mahidol of Songkla, the king's father, to pray for a speedy recovery.
"I have been praying every night asking all holy spirits in the universe to help with his quick recovery," said Phakamas Jongkasemwong, a well-wisher from Bangkok, as she stood near a large portrait of the king.
"I am extremely relieved to hear that His Majesty is improving, but better still we wish him to recover enough to return to the palace."
Adding to the uncertainty about Thailand's future is the political unrest that erupted in 2006, when former prime minister Thaksin Shinawatra was ousted by a military coup after being accused of corruption.
His supporters and opponents continue to vie for power, often taking their case to the streets.
Last year, an anti-Thaksin group occupied the prime minister's offices for three months and shut down the capital's two airports for a week.
|Thousands of Thais including monks have been praying for the king's speedy recovery [EPA]
Concerns over the king's health condition also sparked turmoil in the country's stock market following several days of heavy foreign selling.
The market rose 1.9 per cent at Friday's opening but then slipped slightly as morning trading continued.
Thailand's benchmark stock index, hit hard as investors dumped Thai assets, closed 5.3 per cent lower on Thursday after diving more than eight per cent earlier.
"The market's skittishness is traceable to the possibility of a destabilising power vacuum if the monarchy's power diminishes," political risk experts Eurasia Group said in a report.
The head of Thailand's stock exchange urged investors not to panic but acknowledged rumours about the king's health had taken a toll on the market.
"The reason we've seen the market drop over the past two days is mainly because of news over his health condition," Patareeya Benjapolchai told Reuters.
"Investors should not panic and should trade cautiously during this period of time."