Thailand's much loved King Bhumibol Adulyadej, the world's longest-reigning monarch, died Thursday. He was 88.

Thailand's Royal Palace said Bhumibol died in a "peaceful state" at Siriraj Hospital in Bangkok.

U.S.-born Bhumibol, who was on the throne for 70 years, spent the last several of them in the hospital with a variety of ailments, and his poor health has remained a cause of concern as the question of succession has cast a shadow over Thai political and social life.

Bhumibol was the richest monarch in the world, with a 2014 wealth valued at $30 billion by Forbes.

His son, Crown Prince Maha Vajiralongkorn, 64, will be the new king in accordance with Thailand's constitution, though he does not have his father’s popularity.

In a country beset by political divisions, Bhumibol is seen as a semi-divine figure who embodies the unity and continuity of the nation. His image is ubiquitous around the country — from large public billboards to framed photos in homes and shops.

Hundreds of Thais have gathered around Siriraj Hospital in Bangkok since Sunday, when the Royal Palace made a rare announcement that the king’s health was "unstable."

Thailand’s deeply revered monarch anchored his nation through violent upheavals and decades of economic challenges by devoting himself to the welfare of his people. AP

Born in Cambridge, Mass., and educated in Switzerland, Bhumibol ascended to the throne on June 9, 1946, at the age of 18 following the gunshot death of his brother — King Ananda Mahidol — under circumstances that remain murky. He was the ninth king in the Chakri Dynasty, which dates back to 1782.

Bhumibol strengthened the role of the monarchy in Thailand, after it had been weakened by the end of absolute monarchy in 1932. Thailand's current ruling military junta, led by retired general Prime Minister Prayuth Chan-ocha, has held power since a May 2014 coup.

He is known for his efforts to develop Thailand's rural areas and economy and for forging close ties with the nation's powerful business and military elites.