Thai Prime Minister Prayuth Chan-ocha (left) offers alms to Buddhist monks during a ceremony at the Grand Palace to commemorate Thailand's King Bhumibol Adulyadej's 70th anniversary on the throne, in Bangkok, June 9, 2016. REUTERS/Jorge Silva

Thailand's King Bhumibol Adulyadej, the world's longest-reigning living monarch, marked 70 years on the throne Thursday. While Bhumibol continues to fight his illness, the country began celebrations with a religious ceremony in Bangkok, led by 770 Buddhist monks.

Bhumibol, who ascended the throne at the age of 18, has battled various serious illnesses for more than a decade and has not been seen in public for months. The 88-year-old, who has ruled over a tumultuous period of Thailand's history, is currently in hospital after an operation to widen arteries in his heart.

Thailand's Prime Minister Prayuth Chan-ocha offered alms to Buddhist monks during the ceremony at the Grand Palace in Bangkok Thursday.

On Tuesday, the palace released a statement, saying that the surgery had "satisfactory results." The procedure, known as balloon surgery, was carried out after tests showed he had insufficient blood in his heart muscles.

In recent years, the king, whose official title is Rama IX, has battled flu, pneumonia, intestinal inflammation, hydrocephalus and bleeding in his brain, and his health is closely watched by Thais.

"The relationship between Thais and the King is deep, more than one can actually begin to explain," Col. Winthai Suvaree, a spokesman for the royalist junta, told Reuters. "He is a father to the land."