The revered king died in October 2016 aged 88.
He will be cremated later on Thursday, in a royal pyre to be lit by his son and successor to the throne, King Maha Vajiralongkorn.
The funeral officially started on Wednesday with a merit-making ceremony, a Buddhist rite, in the Grand Palace.
A series of Buddhist rites began in the royal palace on Thursday morning. Later, the royal urn will be moved onto a chariot and brought in a large procession to the site of the cremation.