According to the palace statement, all government buildings will fly the Thai flag at half-staff for 30 days starting October 14, and all civil servants have been ordered to wear black clothing for a year as a sign of mourning, the palace said.
"I would like to ask all Thais to listen to the credible source of news and invite all Thais to dress in a way to mourn his passing for one year to offer our condolences," the prime minister said in a television address.
He further urged citizens to remain calm, for the sake of the country's stability, and said soldiers would be stationed in "every area throughout the kingdom" to boost security.
"Do not let anyone seek an advantage during this time of crisis," Prayut said.
Even on Bangkok's famous Khao San Road -- known for its bright lights, nightclubs and swarms of backpackers -- the mood was somber. Restaurants and food vendors remained open, but had been told by police to close by midnight. The usual loud dance music was absent as nightclubs closed for the night.