The cremation late Thursday will be the biggest Thai royal event for a century and will see colourful funeral processions pass through the capital before his pyre is lit inside a purpose-built site.
Tens of thousands of black-clad mourners have massed along the procession routes near Bangkok's Grand Palace despite heavy overnight rain.
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They gathered for a final chance to be near a king revered as a near-deity during his 70-year reign.
"I want to come and bid goodbye as close as I can to him," Una Tontakulchanchai, 40, told AFP.
Funeral ceremonies began on Wednesday afternoon as Bhumibol's heir King Maha Vajiralongkorn joined a Buddhist merit-making ceremony for his father at the Grand Palace.
Palace aides shuffling on their knees were in attendance on the new king, Rama X of the Chakri dynasty, as he lit candles before the rituals began with monks.
Thailand has been in sombre mood since Bhumibol's death one year ago aged 88, a watershed moment that removed the nation's only unifying figure during decades of political upheaval and economic change.
Many Thais have worn mourning attire for a year as tributes to the monarch dominate every aspect of life.
Public grieving is encouraged by the ultra-royalist junta that grabbed power in 2014.
Any deviation is discouraged by a draconian royal defamation law that punishes criticism of the monarchy with jail time.
On Tuesday soldiers whisked an activist from his Bangkok home to the western province of Kanchanaburi after he threatened on Facebook to disrupt the cremation ceremony.
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Ekachai Hongkangwan, a rare voice of dissent in the junta-ruled nation, was given the option of leaving the capital or staying in a military barracks while the five-day funeral unfolds, according to human rights lawyers.
"He was not charged but the military said he was taken for the sake of order and peace during the royal cremation ceremony," Anon Numpa, an attorney with Thai Lawyers for Human Rights, told AFP.
In a Facebook post last week Ekachai said he would "wear a red shirt and do something unthinkable" on October 26, the day of the cremation.
Authorities said they could not confirm the lawyers' account.
Thursday's cremation ceremony is expected to draw a quarter of a million Thais to central Bangkok, as well as a roster of foreign royals and heads of state.
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