The cabinet has declared Friday a government holiday, and flags are to fly at half-mast for the next 30 days.
The government has also asked for people to wear black, and avoid "joyful events" during this period.
News websites have turned their pages black and white, while tributes have been paid by leaders around the world.
One person in Bangkok told the Associated Press: "There is no word to explain my feeling right now."
"I lost one of the most important people in my life. I feel like I haven't done enough for him. I should have done more," said Gaewkarn Fuangtong.
Thousands of mourners are expected to line the streets of Bangkok on Friday afternoon when the king's body is moved from Siriraj Hospital to the Grand Palace.
The Crown Prince will conduct the bathing ceremony of the king's body - a traditional Thai Buddhist funeral rite - on Friday evening, said palace officials.
Obituary: King Bhumibol
A life in pictures
King Bhumibol's reign - in brief
How King Bhumibol shaped modern Thailand
A night of mourning in Bangkok