The head of Thailand's military junta called for vigilance yesterday following the death of King Bhumibol Adulyadej, the world's longest serving monarch.
His chosen successor asked for a delay before he is proclaimed king, a move likely to add to concerns.
Crown Prince Maha Vajiralongkorn reportedly told Prime Minister Prayut Chan-ocha that he needed time to "mourn, together with the people of Thailand".
"When the appropriate time comes, the Crown Prince said he is already aware of his duty," Prayuth said. "I hope everyone will understand and not cause chaos."
The Prime Minster told the Thai people in a televised address that they "will need to be alert ... to ensure safety".
Security was stepped up in Bangkok's old quarter of palaces, temples and ministries with soldiers at checkpoints, government offices and intersections.
Bhumibol was named king on the day his brother died in 1946. His own death at 88 years old, after 70 years on the throne, prompted deep mourning, but has also cast a shadow over Thailand's political future. He passed away peacefully just before 4pm on Thursday local time at Bangkok's Siriraj Hospital, the palace said.
The Crown Prince does not command the same deep affection in the country and many would like one of the King's siblings to take the throne.
Vajiralongkorn spends much of his time abroad and has shown little interest in politics. The thrice married playboy is known for his erratic behaviour, notably giving his pet poodle Foo Foo the title air chief marshal.
Thailand's military seized control in 2014 and many believe they acted to ensure the generals were in charge when a succession took place.
The Government strictly enforces Thailand's lese majeste laws, which prohibit discussion of the King's health and succession plans. Prosecutions have risen dramatically since the military seized power.
Dr Liam McCarthy-Cotter, an expert on southeast Asia at Nottingham Trent University, said: "There are many who are nervous of this [succession] and will be pushing for an alternative."
King Bhumibol's health had been ailing for the past two years, during which he has rarely been seen in public. But he had been seriously ill in hospital for about a week. Hundreds had been praying on the streets, holding his picture and wearing pink and yellow, colours associated with the monarch.
Many started to cry when his death was announced. He was revered by much of the population, and seen as a figure of stability during decades of coups and political turmoil. The Prime Minister said Thailand had begun a year of mourning and entertainment must be "toned down" for a month.
Meanwhile, tourists in Thailand are being urged to "behave respectfully" and wear "sombre" clothing as the country mourns.
Britain's Foreign and Commonwealth Office updated its travel advice to urge tourists to "respect the feelings and sensitivities of the Thai people at this time".
"Access to entertainment, including restaurants, bars, and shopping areas may be restricted and you should behave respectfully when in public areas," it said.
"If possible, wear sombre and respectful clothing when in public."
The King's death makes Queen Elizabeth the world's longest-reigning monarch.