King Bhumibol, who was widely respected across Thailand, passed away on October 13 at the age of 88 after 70 years as head of state.
Born in Cambridge in the US, he had acceded to the throne in 1946 after his brother, King Ananda Mahidol, died, and guided Thailand through numerous coups and 20 Constitutions.
Authorities said more than 150,000 people occupied Sanam Luang grounds much ahead of the scheduled afternoon programme in remembrance of the late King.
A 100-piece orchestra and a 100-member professional choir conducted by Somtow Sucharitkul accompanied the mass of amateur voices from the people offering their most heartfelt tribute.
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Most of the people carried portraits of the late King and many wept as they sang the royal anthem.
Crowds assembled calmly and followed directions as they assembled near the majestic Grand Palace.
Most had arrived via shuttle buses arranged for the occasion from shopping centres and other public sites on the periphery of the city.
Later tonight participants will hold lit candles and sing the anthem without accompanying music.