It was one of the pivotal moments in Thailand's troubled political history -- with deadly street clashes rocking Bangkok, the country's revered king summoned military and protest leaders who crawled towards him on their knees and made peace.

Two decades after that spectacular arbitration, political foes on opposites sides of the country's deep divide are once again waiting to see if King Bhumibol Adulyadej will seek to calm violence sparked by rallies aimed at toppling Prime Minister Yingluck Shinawatra.

The two camps will be listening closely for any message when the monarch delivers his birthday speech from his seaside palace in Hua Hin on Thursday as he turns 86, bringing a temporary lull in hostilities on the streets of Bangkok.

A dramatic public intervention like the one seen in 1992, experts say, is unlikely to be repeated by the ageing king, who has suffered from a range of ailments in recent years and has been largely out of the public eye.

On that occasion, his arbitration ended clashes between the army and pro-democracy demonstrators that left dozens dead.

This time Bhumibol, who is treated as a near-deity by many Thais, may "ask for calm and reason", said Paul Handley, author of the biography "The King Never Smiles", which was banned in Thailand.

"To do more than that, he can only ask the demonstrators to pull back, which essentially would support the Yingluck government -- not something the palace wants to be seen as doing," added Handley, who now works as an AFP reporter outside Thailand.

The protesters, who include many royalists, are fiercely opposed to Yingluck and her brother, former premier Thaksin Shinawatra who remains a hugely divisive figure seven years after he was ousted by generals loyal to the palace.

The billionaire tycoon-turned-politician is reviled by the elite, Bangkok's middle class and southerners, who see him as corrupt and a threat to the monarchy.

But he is adored by many outside Bangkok, particularly in his stronghold in the nation's north and northeast, for his populist policies targeting the rural poor.

Thaksin or his allies have won every election for more than a decade, and his overthrow triggered years of political turmoil.

Staying above the political fray

Bhumibol's more than six-decade reign has spanned over 20 prime ministers, and almost as many coups -- some unsuccessful.

Helped by well-publicised rural development projects and shielded by harsh defamation laws, the quietly-spoken, bespectacled king enjoys an image of a benevolent moral force in a kingdom with a long history of instability.

He was admitted to hospital in 2009 for treatment of a respiratory condition and stayed there for several years before moving to his coastal palace in August.

Bhumibol remained largely silent in 2010 when deadly clashes between pro-government military forces and pro-Thaksin "Red Shirt" protesters left more than 90 people dead and nearly 1,900 wounded.

"By staying above the Yellow Shirt-Red Shirt fray, he remains above the current political acrimony," said Paul Chambers, director of research at the Institute of South East Asian Affairs at Chiang Mai University.

In 2008, when the royalist "Yellow Shirts" seized Bangkok's major airports and the government headquarters, it was the courts that intervened, with two premiers close to Thaksin subsequently removed by judicial rulings.

With political tensions once again gripping the capital, Thailand's military has shown little enthusiasm for intervening to seize power, mindful of the years of tensions and sometimes violent street protests that followed the 2006 coup.

Any intervention to end the standoff is only likely through the judiciary or perhaps the military, Chambers said.

But a coup "would be highly unlikely when courts can dislodge the legitimacy of Yingluck", he added.

The recent protests were triggered by an amnesty bill, since abandoned by the ruling party, which opponents feared would have allowed Thaksin to return from self-exile without serving jail for a corruption conviction he contends is politically motivated.

His opponents are appalled by the idea of the return of such a polarising figure, particularly as the country quietly braces for the eventual end of Bhumibol's reign.

"Behind the scenes in all of this turmoil since the early 2000s is the succession," Handley said.

"Thais disagree about how the country should prepare for this -- essentially, who should be running the country when it comes," he said. "The military will be key in maintaining stability when succession takes place."