He became prime minister reluctantly in 1980, and stayed at the helm for eight years, guiding the country through economic problems and a series of military challenges, including two coup attempts.

At a time when Thailand was a front-line state in the Cold War, and seemingly threatened by Soviet-backed Vietnamese expansion, Prem kept the country on a pro-West course, but also forged closer relations with China.

He came up through the ranks of the powerful military and first achieved national prominence in 1974 when, as army commander in Thailand's rural northeast, he favored development and civic action instead of military might against communist insurgents. He later became army commander-in-chief and defense minister before parliament installed him as prime minister as the only viable candidate at a time of political turmoil.

In later life, his career has been defined by his relationship to two men: Bhumibol, to whom he was unswervingly loyal, and Thaksin Shinawatra, a twice-elected prime minister. Thaksin's supporters believe that Prem instigated the coup that removed the populist prime minister from power in 2006.

On one occasion, Prem's house was the focal point for pro-Thaksin protests that turned violent, with demonstrators battling with police late into the night.

Prem turned 96 in August. Prime Minister Prayuth Chan-ocha, who seized power in a 2014 coup, led the Cabinet in wishing the elder statesman well on his birthday.

One of his last public appearances before the death of Bhumiphol was on Oct. 7, when he spoke on one of his keynote subjects, anti-corruption, urging that people need to set an example to the younger generations.

It isn't known how long Prem will remain as regent. (AP)