Thai King Bhumibol Adulyadej sits in his vehicle as he leaves Siriraj hospital for Coronation Day in Bangkok on Tuesday. Photo: AFP

Thailand's revered but ailing King Bhumibol Adulyadej made a rare public appearance on Tuesday, leaving the hospital where he has been convalescing for much of the past few months.

A heavily guarded convoy carrying the wheelchair-bound 87-year-old monarch left Bangkok's Siriraj Hospital for the Grand Palace, the sprawling complex in the heart of the old quarter that is the seat of the Chakri dynasty.

The palace visit was made on Coronation Day, a Thai public holiday that marks Bhumibol's official coronation in 1950, three years after his reign began following the death of his brother.

The appearance of the world's longest serving monarch after a recent no show will be a source of relief for many Thais who revere him as a semi-divine figure.

He had been expected to appear publicly for his birthday in December, a time when the monarch traditionally addresses his people. But the audience was cancelled at the last minute on the advice of his doctors.

Most Thais have only known King Bhumibol 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 divide.

King Bhumibol, formally known as Rama IX, has spent most of the past few months in hospital after undergoing an operation to remove his gall bladder in October.

In the past two months Bhumibol had made two brief trips from his hospital bed, one in early May to a nearby palace and one last month to view the Chao Praya river that runs through Bangkok.

Hundreds of Thais gathered outside of Siriraj on Tuesday, many waving flags and dressed in the royal color yellow, an AFP photographer on the scene said.

When the king's convoy passed many burst into applause and shouted of "Long Live the King!"

Thai television stations showed the monarch seated on a thrown in a hall at the Grand Palace for a ceremony in front of the country's elite, backed by chanting Buddhist monks. He returned to the hospital after the ceremony ended.

Prominent royal figures who attended included the next in line to the throne, Crown Prince Maha Vajiralongkorn, and his sister Princess Sirindhorn.

Prime Minister Prayut Chan-o-cha, an ultra-royalist former army chief who seized power in a coup, also attended.

The military took over in May following months of street protests that led to the toppling of Yingluck Shinawatra's democratically elected government.

It was the latest chapter in Thailand's long-drawn political conflict, which broadly pits a Bangkok-based middle class and royalist elite, against rural and working-class voters loyal to Yingluck and her brother Thaksin, who was also deposed in a coup in 2006.