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.
Exiled Prime Minister is waiting in the wings in a time of uncertainty
Thailand is the land of smiles and military coups. King Bhumibol Adulyadej, the world's longest reigning monarch, witnessed at least ten coups in his 70-year reign, interspersed with brief periods of constitutional semi-democracy.
The 1957 coup rehabilitated the monarchy as the symbolic focus of Thai Buddhist loyalty. As the country modernised into one of the leading South East Asian economies in the 1990s, Bangkok's political, business and academic elites accepted that it prospers "under an authority around which all elements can rally".
Thais emphasise the kamlungjie or awe that the monarchy evokes. To demonstrate loyalty, supporters, including civil servants and TV presenters, wear their yellow shirts to work on royal Friday, and the Government reserves dire penalties for the crime of lese majeste.
Bhumibol was, then, far more than an emblem of constitutional authority. He was the iconic focus for an authoritarian politics much resented in the poor northeast of the country. The Asian financial crisis, which began in Thailand in 1997, witnessed the emergence of the populist Thai Rak Thai party of Chiangmai tycoon Thaksin Shinawatra. His health and agricultural reforms gave him a popular rural base there that has remained loyal despite his removal in yet another military coup in 2006.
The last decade has witnessed an unholy coalition between Bangkok business elites, the middle-class Thai Democrat party, and the royalist military to prevent the return of the exiled Thaksin, despite red shirted Thaksin supporters bringing Bangkok to a halt in 2010 and the election victory of his sister Yingluck the next year. Her Government lasted until the most recent coup, in 2014. General Prayuth Chan-ocha's current government relies for its legitimacy upon the king.
The problem that now faces the junta is the extent to which Crown Prince Maha Vijiralongkorn will command popular and elite loyalty. The prospects are not good. He has a reputation for self-indulgence, not without cause. Intriguingly Thaksin has provided generous support for the crown prince's playboy lifestyle.
The succession and its constitutional fall-out are likely to provide for both interesting and unstable times.