Doctors treating ailing Thai King Bhumibol Adulyadej said his condition was not stable but they were aggressively treating his symptoms.
Thai prime minister Prayuth Chan-ocha cut short a trip to the country's east and returned to Bangkok as the 88-year-old monarch's condition remained critical, even as crowds swelled outside the Siriraj hospital where Bhumibol is being treated.
The royal doctors said the king was on a ventilator and that they were performing continuous renal replacement therapy (CRRT) while also administering drugs to control blood pressure, the Bangkok Post reported.
"Blood tests show he has an infection and his liver is working irregularly," a statement from the royal household bureau said.
The government said the prime minister rushed back to the capital from the from Chon Buri province to hold an emergency meeting with heir apparent Maha Vajiralongkorn.
Vajiralongkorn, who also flew back to Thailand from Germany, visited the hospital on Wednesday afternoon. Princess Chulabhorn and her daughters HRH Princess Siribha Chudhabhorn and Princess Adityadornkitikhun were also at the hospital.
As uncertainty over the health conditions of the much revered king rose, visibly worried citizens thronged in front of the hospital with prayers.
On Sunday, the Royal Household Bureau had said the doctors had performed haemodialysis on His Majesty, who has been in and out of the hospital for many years.
Thais turned out in bright pink clothes which is considered as auspicious colour that will attract good luck for the king.