BANGKOK (REUTERS) - Thailand's revered King Bhumibol Adulyadej made a rare public appearance on Monday (Dec 14) after failing to show up for nationwide celebrations marking his birthday earlier this month.

Local television showed footage of the monarch, who turned 88 on Dec 5, swearing in dozens of judges at a reception room in a Bangkok hospital where he has been convalescing.

The King was last seen in public on Sept 1 and did not make an appearance on his birthday - the second year running he missed the celebrations he would usually take part in.

He has spent the past few months at Siriraj Hospital where he was treated for hydrocephalus, a build-up of cerebrospinal fluid that surrounds the brain.

The condition of the King, the world's longest serving monarch, is of huge public interest but cannot be discussed openly in the kingdom because the royal family is shielded by one of the world's toughest lese majeste laws.

Since the military government took power following a 2014 coup, prosecutions of those accused of having defamed the royals have increased dramatically.

Article 112 of the country's criminal code carries up to 15 years in jail for each count of insulting the king, queen, heir or regent.

Last week, thousands of cyclists led by Crown Prince Maha Vajiralongkorn rode through Bangkok to honour the King in an outpouring of loyalty that comes against the backdrop of the unprecedented crackdown on those perceived as critical of the monarchy.

Two suspects have died in custody during a police investigation of corruption involving the "Bike For Dad" event and another cycling event aimed at celebrating Queen Sirikit.

A number of people were accused of falsely claiming close royal connections to raise funds for the cycling events.

The police last week said they had launched an inquiry into US Ambassador to Thailand Glyn Davies after he criticised "lengthy and unprecedented" jail sentences for those found guilty of lese majeste.

The Crown Prince does not command the same devotion as his father does, leading to some apprehension about the succession which is seen as an aggravating factor in Thailand's bitter political divide.

The King, who wore a dark suit and colourful tie, did not speak during the undated footage of the ceremony which official royal news said took place on Monday afternoon.

Notices on the King's health come from the Royal Household Bureau, which tightly controls news about the royal family.