Large queues formed at the Grand Palace in the Thai capital as mourners paid their respects to King Bhumibol Adulyadej, who died on Thursday.
Thousands, dressed in black, waited to enter to sign a book of condolences at the palace in central Bangkok.
Free buses were laid on to transport mourners from rural areas.
A regent will stand in until the late king's son, Crown Prince Maha Vajiralongkorn, accedes to the throne.
Prince Vajiralongkorn has asked for a delay while he grieves for his father, who at 88 was the world's longest-reigning monarch.
Speaking on state television late on Friday, Deputy PM Wissanu Krea-ngam said the head of Thailand's privy council - currently former Prime Minister Prem Tinsulanonda - would be regent.
"The situation will not be used for long," Mr Wissanu said.