Thailand's revered 86-year-old King Bhumibol Adulyadej has had his gall bladder removed, the palace said Monday (Oct 6), two days after he was rushed to hospital.

The world's longest-serving monarch was driven from his coastal palace to Bangkok's Siriraj hospital with a fever on Friday evening.

Tests over the weekend found he had a swollen gall bladder, prompting the operation to remove the organ late Sunday, the Royal Household Bureau said in a statement.

Doctors were "satisfied" with the outcome of the surgery and the king's condition was "improving" early Monday, the statement has added. "His majesty's heartbeat has slowed down... his blood pressure is in the normal range and his temperature is lower," it said.

The King, officially known as King Rama IX, has suffered from numerous ailments in recent years. He is treated as a near-deity in Thailand and his health is a subject of great public concern.

Surgery to remove the gall bladder is common and patients tend to recovery quickly from the operation.

The king had left Siriraj hospital only last month after a stay of almost six weeks for a check-up. He lived in the same hospital for nearly four years after being admitted with respiratory problems in 2009. However, there was no explanation from the palace over his prolonged stay.

Born on December 5, 1927, the king came to the throne aged 18 in 1946. After finishing his studies in Switzerland, he was crowned in 1950.

As a Constitutional Monarch, the king has no official political role. However, Thais see him as a unifying figure and a moral force during a reign that has been marked in recent years by political turmoil. Therefore, to the Thai people he is above politics. The king remains extremely popular and revered in the country.

In August he formally endorsed the kingdom's new Prime Minister, Prayuth Chan-Ocha - who seized power from the elected administration on May 22, shortly after Yingluck Shinawatra was booted out of office by a controversial court ruling. Prayuth visited the hospital early Monday to sign a book of support for the monarch.