BANGKOK - Thailand's ailing King Bhumibol Adulyadej cancelled plans to hold a public annual audience marking his 87th birthday Friday on his doctors' advice.

The world's longest-serving sovereign has been seen by Thais as a unifying father figure throughout a turbulent six-decade rule, and his absence comes at a time of profound concern over the kingdom's future as his reign enters its twilight years. The king, formally known as Rama IX, has spent most of the last few months in hospital after undergoing an operation to remove his gall bladder in October but had been expected to appear publicly for his birthday.

The last minute cancellation of celebrations came after doctors advised him against appearing, the Royal Household Bureau said in a statement early Friday.

A palace official told AFP plans for a grand audience and live broadcast were subsequently scrapped.

The king's no-show comes six months after the military toppled the elected government in a coup and amid a high profile investigation into police corruption which has led to the recent arrest of relatives of Princess Srirasmi, the wife of Crown Prince Maha Vajiralongkorn.

May's coup was the latest chapter in Thailand's long-drawn political conflict, which broadly pits a Bangkok-based middle class and royalist elite, backed by parts of the military and judiciary, against rural and working-class voters loyal to the family of deposed premier Thaksin Shinawatra.

Experts said the monarch's absence - while not entirely unexpected given his ill-health - was significant.

"In time of political crisis, many Thais would rely on the wisdom of the King and his speech would be meaningful," Pavin Chachavalpongpun, an associate professor at Japan's Kyoto University, told AFP.

The king's birthday is a public holiday and is also known as "Father's Day", a reflection of the Thai view that their monarch is the father of the nation.

Hundreds of people, many dressed in the royal yellow colour and waving flags, gathered in the grounds of Bangkok's Siriraj Hospital, where King Bhumibol is convalescing, to chant "Long Live the King!", an AFP reporter at the scene said.

"I wish him good health and that he stays as the soul of the Thai people for a long time," said well-wisher Nonglak Thongliam.

Doctors treating the king said the monarch was not in a serious condition and that he simply needed time to recover from a fever.

"The king is not sick or seriously ill. He is recuperating to regain his strength," Udom Kachintorn, the dean of the Faculty of Medicine at Siriraj Hospital, told reporters.

Helped by well publicised rural development projects - and shielded by one of the world's harshest royal defamation laws - King Bhumibol is revered by many Thais as a semi-divine figure and a benevolent moral force in a kingdom with a long history of instability and episodes of political bloodshed.

Most Thais have only known one king on the throne and anxiety over the future once his six-decade reign ends is seen as an aggravating factor in Thailand's bitter political schism.

While King Bhumibol has endorsed all the successful coups that permeated his reign, he also dramatically intervened at key moments in the 1970s and 1990s to halt further violence.

But he has remained silent in public on the latest army power grab which brought former general Prayut Chan-O-Cha to power in May.

Thailand remains under martial law, a condition the arch-royalist army says is necessary to keep the peace as it vows to expunge the kingdom of corruption and reboot Thai democracy.

Critics say the military used its status as the defender of the monarchy as a pretext to seize power from the elected government aligned with Shinawatra - the billionaire self-exiled former premier whose emergence as an electoral force in 2001 shook up Thai politics.

Despite staunch resistance from the establishment Thaksin-affiliated parties have swept every general election since 2001.

In a report published this week International Crisis Group said the coup was "more likely to bring conflict than cohesion" with a military "gelding elected leaders in favour of unelected institutions".

"A failure to fix this dysfunction risks greater turmoil," the report warned.