Thailand's revered King Bhumibol Adulyadej, the world's longest-reigning monarch, was being treated in hospital Monday after being admitted with a fever and fatigue, hospital sources said.
The 81-year-old king went to a Bangkok hospital on Saturday night to receive an intravenous saline drip and antibiotics, the royal household bureau said in a statement on Sunday night.
Dozens of Thais went to Siriraj hospital on Monday to sign a book sending their best wishes for the king to get well soon, an AFP photographer said. Some left flowers for the elderly monarch.
A hospital source speaking on condition of anonymity confirmed that King Bhumibol was still being treated at Siriraj but said the royal household was expected to release further details later.
The household statement on Sunday said that the king had "fever, fatigue and loss of appetite. The medical team invited him to go to Siriraj hospital to find the cause and get medical treatment."
"His Majesty has been given intravenous fluids and been provided with antibiotics," it added.
Prime Minister Abhisit Vejjajiva said Sunday that he was aware the king had gone to hospital but insisted it was only for a check-up, adding: "His Majesty's condition is not a problem."
King Bhumibol has long projected an image as the only unifying figure in a nation that has seen 23 prime ministers, 17 constitutions and several coups during his 63-year reign.
While remaining a constitutional monarch, he waded into politics during the 1970s and in 1992, when he ordered military dictatorships to end bloody crackdowns.
Thai children are taught that respect for the king; patriotism and religious devotion are closely intertwined, giving him an almost divine status in Thailand's Buddhist society.
But strict lese majeste laws under which anyone who insults the royal family can be jailed for up to 15 years also protect the monarchy.
Last month the king issued the latest in a series of warnings since a September 2006 coup ousted then-prime minister Thaksin Shinawatra that the country could "collapse" if Thailand's feuding political factions do not unite.
Thousands of protesters took to the streets on Saturday, the third anniversary of the coup, to listen to a video speech by the exiled Thaksin.
King Bhumibol was diagnosed as suffering from "mild fever" in December as he celebrated his birthday, causing him to cancel his annual birthday eve speech.