Well-wishers stand weep outside Thailand's King Bhumibol Adulyadej at the Siriraj hospital where he is residing in Bangkok, Thailand, October 13, 2016. [Photo/Agencies]

The king stepped in to calm crises on several occasions during his reign and many Thais worry about a future without him. The military has for decades invoked its duty to defend the monarchy to justify its intervention in politics.

Military government leader Prime Minister Prayuth Chan-ocha said the country was in "immeasurable grief ... profound sorrow and bereavement".

He said security was his top priority and called for businesses to stay active and stock investors not to dump shares. Banks and financial markets are to stay open on Friday, industry officials said.