Thailand's revered king on Thursday failed to deliver his traditional birthday speech, dashing Thai hopes that the address would help resolve the country's political paralysis and unify a divided nation.

King Bhumibol Adulyadej, who turns 81 on Friday, was unable to deliver the speech "because he was a little sick," his son, Crown Prince Vajiralongkorn, told dignitaries gathered at the Dusit Palace to hear the king. The comments were broadcast live on radio.

While the king is known to be unwell, he was not expected to miss such an important occasion. The news came as a shock in this Southeast Asian nation of 63 million people, who revere the king as a selfless humanitarian. Many people wear yellow, the king's color, once a week as a mark of respect and affection for the beloved monarch.

"The king has said to thank (you) for the wishes given out of loyalty. He wants to return the good wishes. He wants everyone to have strong mental and physical health to perform their duties for the public," Vajiralongkorn said.

Many Thais were eagerly awaiting the king's speech, hoping to receive guidance on how Thailand can resolve its political crisis triggered by an anti-government group whose members seized two main airports for a week.

The protest group, the People's Alliance for Democracy, ended its siege Wednesday after a court ruling ousted Prime Minister Somchai Wongsawat and disbanded the top three parties in the ruling coalition for committing electoral fraud. But they have vowed to resume their campaign if Somchai's proxies return to power.

Bhumibol, the world's longest-serving monarch who has reigned since 1946, is a constitutional monarch and has no role in government but has stepped in several times in the past to resolve political crises.

The king's ill-health "can only deepen ... concern about the fact that an era about which they felt really secure is approaching its end," said Michael J. Montesano, an assistant professor at the National University of Singapore.

"It's going to contribute to feelings of great dread," he told The Associated Press.

Bhumibol was hospitalized last year for symptoms of a stroke and a colon infection. He also has a history of heart trouble and was operated on in 2006 for a spinal problem.

The king has never publicly commented on his successor, an issue that weighs heavily on Thai minds even if it is rarely discussed in public.

Vajiralongkorn, 56, lacks the stature and popularity of his father. There is great concern that the prince, who has married three times and fathered seven children, would have difficulty living up to Bhumibol's record of hard work and diligence.

The king's annual birthday speech is usually delivered in front of senior government officials and other guests representing different sectors of society. It is broadcast live on national radio and later on television.

But after a one-hour delay, the radio announced that Vajiralongkorn would take the king's place at the function. The crown prince spoke for only three minutes to inform the nation of his father's condition.

Immediately after the short address, his sister, Princess Sirindhorn, spoke to the nation to say the king was suffering from bronchitis and inflammation of the esophagus.

He has no fever but needs to regain strength before coming out in public, she said. "His condition is not serious. He is weak," she said.

The king's last public appearance was on Tuesday when he looked haggard and weak while inspecting a guard of honor by royal troops. He spoke briefly, reading hoarsely from a text.

The royal crisis could not have come at worse time for Thailand as it reels from the anti-government movement by the People's Alliance for Democracy.

It started with a campaign in late 2005 to oust then Prime Minister Thaksin Shinawatra, who was eventually removed in a 2006 military coup amid accusations of gross corruption, abuse of power and attempting to undermine the monarchy.

Thaksin's supporters won elections held under military rule in December 2007. The alliance rejected the outcome, saying the new government was Thaksin's proxy.

On Aug. 26, alliance members stormed and occupied the grounds of Government House, which houses the prime minister's office.

They also occupied the international Suvarnabhumi airport and the domestic Don Muang airport last week to prevent Prime Minister Somchai from returning to Bangkok from a summit in Peru.

Although Somchai and other party leaders were banned from politics from five years in Tuesday's court ruling, other members of his coalition are preparing to join new parties and choose a new prime minister, a move that could once again prompt the protest alliance to take to the streets if the next leader is seen as a reincarnation of the Thaksin government.

Rojana Duangkaew, 28, a pharmacist who attended many of the alliance's protests, said the king is "probably sick with worries about the situation in the country."

"The country is sick so the king is sick. I think all Thais are worried. Thailand needs him. He is the only one who can make people on both sides realize they are ruining the country. He is the only one who can unify Thailand," she said.

The alliance says the rural poor — who form the main power base of the Thaksin group — are uneducated and susceptible to vote buying. It wants a system in which a majority of lawmakers would be nominated by professional and social groups instead of being elected.

Thaksin, who has fled overseas, has been sentenced to two years in prison on an abuse of power charge. The former telecommunications billionaire remains highly power among poor rural residents for his populist policies.

On Thursday, authorities said Suvarnabhumi airport, Thailand's main international gateway and a regional hub, would be "open for full services including check-in and immigration" on Friday.