BANGKOK - Thailand's revered King Bhumibol Adulyadej called for national unity on Wednesday at joyous celebrations on the 80th birthday of the world's longest reigning monarch.

King Bhumibol told hundreds of dignitaries gathered at Bangkok's Grand Palace, and many millions more glued to their televisions around the country, that he wished "for all Thais to unify and perform their duty to support to each other".

"If everyone maintains unity ... our country can be stable, develop and make progress," said the bespectacled monarch, clad in an ornate coat of gold thread, 61 years after his coronation.

His comments echoed those he made in a nationally televised address on Tuesday, showing the King's clear unease at the political instability since last year's coup against Prime Minister Thaksin Shinawatra.

Many analysts say the December 23 election is likely to lead only to further polarisation between Thaksin supporters in the countryside and the royalist military establishment and Bangkok middle class that ousted him.

In the worst-case scenario, some analysts see Thaksin's friends and foes facing off in the streets, making further military intervention inevitable.

After his 3-minute speech, the monarch, with Queen Sirikit and Crown Prince Maha Vajiralongkorn alongside him, was greeted with shouts of "Long Live The King" from the dignitaries.

Outside the palace, tens of thousands of Thais wearing yellow also cheered and waved national and yellow flags. Yellow is the colour of Monday, the day of the King's birthday.

"I am very proud that we have a father like him. He protects our country and our people and makes us happy," 83-year-old Samai Wuthkaew said as she held a photograph of the King.

HEALTH CONCERNS

The 55-year-old Crown Prince and heir apparent began the ceremony with a speech wishing his father well on his birthday.

King Bhumibol, a semi-divine figure whose portrait is hung in almost every Thai home, enjoys almost universal respect as a champion of the poor and the environment.

In his speech on Tuesday he joked about his recent treatment for a blood clot in his brain and an intestinal problem.

But the three-week hospital stay renewed fears about the health of the monarch who has been treated for a range of ailments in recent years, including a spinal operation last year.

"People love the King and they don't want to talk about his passing because they love him. They want him to be around forever," a political analyst said.

"The King is supremely revered in Thailand and his successor will have big shoes to fill," the analyst said.

That reverence coupled with tough lese majeste laws, under which insulting the monarchy can be punished by 15 years in jail, have discouraged critical debate about the royal succession and the role of the monarchy in society.

Despite being a constitutional monarch, King Bhumibol has made several forays into politics during 61 years on the throne which have witnessed 18 military coups. He has variously come out in support of democratic and military governments.

Thaksin, now living in exile in London and facing corruption charges he denies, won landslide election victories in 2001 and 2005 and is still hugely popular in the countryside where the majority of Thais live.

The People Power Party, which his supporters took over after Thaksin's party was banned, is likely to win the most seats in the poll but fall short of an absolute majority, analysts say.