An emotional crowd held a late-night vigil in Bangkok for the king, who died on Thursday aged 88.
His body is to be moved to the city's Emerald Temple later on Friday. The official mourning will last a year.
Crown Prince Maha Vajiralongkorn is to be the new monarch, but has asked for a delay before taking over the role.
Flags are to fly at half-mast for 30 days and the government has asked for "joyful events" to be halted for 30 days.
King Bhumibol was seen as a stabilising figure in a country hit by cycles of political turmoil and coups.