Thailand's King Bhumipol Adulyadej, 81, failed to make his traditional address to the nation due to ill health, marking the first occasion in the past six decades he has failed to address the country for Thailand's national day.

Thailand's King Bhumibol Adulyadaj and Queen Sirikit review the Royal Guards in Bangkok, 02 Dec 2008

Instead, his son, Crown Prince Vajiralongkorn was on national radio to inform the country his father was unable to deliver the speech because he was a little unwell.

The crown prince told listeners the king wanted to thank them for their well wishes and called on the people to have strong mental and physical health in the performance of their duties.

The crown prince's sister, Sirindorn sought to reassure the general public the King's condition was "not serious", but he was weak.

The annual address had been widely looked to as the king has regularly made calls for strengthening national unity during the country's often turbulent political climate.

King Bhumipol, highly revered in the country, has been in declining health.  In 2006 he underwent corrective spinal surgery.

This week's address had gained greater importance in light of the recent political crisis, its most severe since 1992.  

Uncertainties remain among leading parties in selecting a new prime minister.  Earlier this week a constitutional court dissolved the previous governing People's Power Party, forcing Prime Minister Somchai Wongsawat to resign.   

Anti-government protesters collect helmets, sticks, other weapons as they dismantle checkpoint at Suvarnabhumi airport, in Bangkok, 03 Dec 2008

The resignation follows six months of protests by the People's Alliance for Democracy, a strong supporter of the constitutional monarchy.

In recent heightened tensions, the People's Alliance for Democracy seized Thailand's major airports in Bangkok.  But airports began moving back into operation after the week-long siege ended Wednesday.  The protest had led to a major disruption for more than a quarter-of-a-million people, costing the vital tourism industry millions of dollars.

Meanwhile, the rights groups Human Rights Watch and Amnesty International called for an end to political violence in Thailand that has claimed the lives of at least seven people and left hundreds injured during the recent months of anti-government protests.