The world's longest-serving head of state had a sore throat with an infection, which made it difficult for him to eat and speak, Crown Prince Maha Vajiralongkorn said on national radio.

He is on a saline intravenous drip to receive fluids because he cannot eat, said Crown Princess Maha Chakri Sirindhorn.

Many of of Thailand's 65 million people anticipated the revered king, who turns 81 Friday, would use his nationally broadcast speech to indicate his feelings about the People's Alliance for Democracy, which occupied Bangkok's two main airports for a week demanding Prime Minister Somchai Wongsawat resign, the Los Angeles Times reported.

The protesters lifted their crippling siege Wednesday after Thailand's Constitutional Court disbanded three parties in Somchai's coalition government for electoral fraud in last December's parliamentary elections and forced the premier to step down.

Bhumibol's children mentioned nothing about politics.

The monarch, who has ruled for 62 years, has no formal political power, but has made decisive interventions in Thai politics when Thailand was in turmoil.

He facilitated Thailand's transition to democracy in the 1990s and used his considerable influence to stop coups in 1981 and 1985.