King Bhumibol is the world's longest reigning current monarch
The 81-year-old king of Thailand has been admitted to hospital suffering from a fever.
Doctors said King Bhumibol Adulyadej, the world's longest-serving monarch, had shown signs of fatigue and was being treated with antibiotics.
King Bhumibol is deeply revered by most Thais and his health is a matter of public anxiety.
Prime Minister Abhisit Vejjajiva told reporters there was "nothing to be concerned about".
People have gathered at the hospital to offer prayers and convey their good wishes.
A statement from Thailand's royal household said King Bhumibol was taken to the Siriraj hospital in Bangkok on Saturday night.
The Royal Household Bureau said the king was suffering from a fever, fatigue and loss of appetite. He is being treated with intravenous drips and antibiotics.
Prime Minister Abhisit Vejjajiva said on Sunday that he was aware the king had been admitted to hospital, but insisted it was only for a check-up.
"His Majesty's condition is not a problem," Mr Abhisit told reporters.
King Bhumibol has long been seen as the only unifying figure in a nation that has seen at least 24 prime ministers, 17 constitutions and more than a dozen military coups during his 63-year reign.
He is widely respected among the Thai people - and he is sometimes accorded an almost divine reverence.
The Bangkok Post newspaper quoted a 60-year old woman, Warinan Phurahong, as saying she ran to the hospital when she heard about the king's condition and plans to stay there until His Majesty recovers.
The king, a constitutional monarch, made a rare call for stability and reconciliation in Thai politics last month. Factions for and against the ousted Prime Minister Thaksin Shinawatra are jostling for power, often using the king's name and image to strengthen their case.
The weekend saw another outbreak of street demonstrations from the opposing political camps.
Mr Thaksin's supporters, the "red shirts", marked his ousting in a coup three years ago with peaceful rallies in Bangkok.
Mr Thaksin, criticised by his opponents for not showing enough respect to the monarchy when he was in office, spoke to the crowd from exile by video link. He wants fresh elections and a pardon for a conflict of interest conviction.
Meanwhile, the largely pro-government "yellow shirts" demonstrated on the Thai border with Cambodia in a long-running sovereignty dispute over a temple complex that straddles the boundary.
Their protests ended in violent clashes with Thai police.
Thailand remains deeply divided three years after the 19 September 2006 coup which drove out Mr Thaksin while he was in New York to attend the United Nations General Assembly.
Mr Abhisit is on his way to the same event, but the chief of the kingdom's powerful army has scotched rumours that there would be another putsch in his absence.