Thousands of Thais have gathered at Bangkok's royal palace for late-night vigils to mourn King Bhumibol Adulyadej, who died on Thursday.
His body was brought to the palace in a convoy as mourners lined the streets weeping. Millions watched live on TV.
Crown Prince Maha Vajiralongkorn has been named as successor, but has asked for a delay in the process. A regent has been appointed.
King Bhumibol was the world's longest-reigning monarch.
He had been ill for a long time and died on Thursday. Official mourning will last a year.
The cabinet declared Friday a government holiday, and flags are to fly at half-mast for the next 30 days.
People have been asked to wear black, and avoid "joyful events" during this period. Cinema screenings, concerts and sports events have been cancelled or postponed.
News websites have turned their pages black and white. All television channels in Thailand aired state media programmes including live coverage of the day's events.
The crown prince travelled in the convoy carrying the king's body to the Temple of the Emerald Buddha in the Royal Palace.
"We no longer have him," AFP news agency quoted Phongsri Chompoonuch, 77, one of those who lined the streets for the procession, as saying.
"I don't know whether I can accept that. I fear, because I don't know what will come next."
After the convoy arrived at the temple, the crown prince conducted the bathing ceremony of the king's body, a traditional Thai Buddhist funeral rite.
Buddhist monks then said prayers over the king's remains. His body will be on display for people to pay their respects.
Thousands of people have now gathered at the palace complex.
It could be months before the late king's cremation.
King Bhumibol earned the devotion of Thais for his efforts to help the rural poor, including agricultural development projects and works of charity.
The monarch was also seen as a stabilising figure in a country often wracked by political turmoil.
Thailand remains under military rule following a coup in 2014.
The country has suffered from political violence and upheaval over the past decade, as well as a long-running Muslim separatist insurgency in the southern provinces which sees regular small-scale bomb attacks.
Though a constitutional monarch with limited official powers, many Thais looked to King Bhumibol to intervene in times of high tension. He was seen as a unifying and calming influence through numerous coups and 20 constitutions.
However, his critics argued he had endorsed military takeovers and at times had failed to speak out against human rights abuses.
The crown prince, who is 64, is much less well known to Thais and has not attained his father's widespread popularity. He spends much of his time overseas, often in Germany.
While Prime Minister Prayuth Chan-ocha has said the crown prince will ascend the throne next, there is uncertainty over when that will happen.
Mr Prayuth said the crown prince had asked for a delay in the succession as he wanted time to mourn with the nation.
Privy Council President Prem Tinsulanonda, a 96-year-old former prime minister, has been named regent in line with the constitution.
He remains as regent until the Thai assembly invites the heir to succeed to the throne, the military government has confirmed.
Strict lese-majeste laws protect the most senior members of Thailand's royal family from insult or threat. Public discussion of the succession can be punishable by lengthy jail terms.
Given the pivotal role the king has played in maintaining the balance of power in Thailand's volatile political environment, the succession will be a formidable challenge for the government, says the BBC's Jonathan Head in Bangkok.