By Amy Sawitta Lefevre
BANGKOK (Reuters) - Thais marked the birthday of King Bhumibol Adulyadej, the world's longest-reigning monarch, early on Saturday, by giving alms at temples around the country.
Celebrations in Thailand, where the monarch's birthday is also national Father's Day, come amid a widening police investigation into a group of people charged with insulting the monarchy.
Ten people, including senior police and military officers, have been charged with a range of offences, including corruption and alleged royal defamation, over their involvement in two nationwide cycling events aimed at celebrating the royals.
Two suspects have died in military custody.
Criticism of Thailand's monarchy is outlawed by draconian lese majeste laws that provide for jail sentences of up to 15 years for each perceived insult to the monarchy.
Since taking power in a May 2014, the ruling junta has cracked down on perceived royal insults and has doled out record jail sentences of up to 60 years for lese majeste offences.
The royal insult investigation comes amid growing concern over the health of the king, who is highly revered by Thais, and the question of succession - an issue which will dominate political developments in the Southeast Asian country.
King Bhumibol, who turns 88, was last seen in public on Sept.1, and officials say he will not be making a public appearance on his birthday.
The king has spent the past few months at a Bangkok hospital where he was treated for 'water on the brain', or hydrocephalus, a build-up of cerebrospinal fluid that surrounds the brain.
He canceled a public appearance to mark his birthday last year, on medical advice.
Crown Prince Maha Vajiralongkorn is expected to carry out ceremonies to mark his father's birthday, including an evening candle-lighting ceremony in Bangkok to pay tribute to the king.
Having reigned for nearly seven decades, King Bhumibol is the only monarch that most Thais have known, and anxiety over the eventual succession is seen as an aggravating factor in Thailand's bitter political divide.
"I have a deep love for him and want him to be healthy,"
said Suchada Charan, a retiree from Bangkok, after giving alms to Buddhist monks in honor of the king's birthday.
"Father's Day in Thailand is the king's day. He is our father," she said.
(Editing by Simon Cameron-Moore)