BBC South East Asia Correspondent Jonathan Head is in Bangkok among mourners.
He said the king's death has been a "long goodbye" for Thailand.
"This is a man who far more than the British queen was seen as the heart of the nation," he said. Many Thais feel an "unspoken emotional bond" with King Bhumibol, despite his long absence from public life, he said.
Widespread mourning throughout the country is expected for months.