The coronation represents a renewal of the monarchy's power after the October 2016 death of King Vajiralongkorn's revered father, King Bhumibol Adulyadej.
It also comes amid more than a decade of political strife, including a 2014 military coup and a contentious election less than two months ago.
The 66-year-old served as king for more than two years since he assumed the throne.
On Saturday, he took the crown from the chief Brahmin, a Hindu priest who has played a guiding part in the mixed Buddhist-Hindu ceremony, during a televised ritual from the Grand Palace.
The 200-year-old, 16lb heavy crown was handed to the king, known as Rama X, as part of the third and final rite of the coronation ceremony, the presentation of royal regalia.
Earlier, the king wore a simple white robe as he entered a small pavilion where he was briefly showered with water from several holy rivers and ponds and other sources.
Other water was poured on him from old royal water vessels. The rite, known as the Royal Purification Ceremony, was accompanied by music from drums, conch shells and other instruments. Outside the palace, artillery was fired in a salute to the monarch.
"This ceremony is significant to Thailand because the monarchy ... is a very important institution of our country and is the soul of our nation," said Naowarat Buakluan, a 41-year-old civil servant.
“If you ask why the ceremony is being held this year when his majesty has already ascended the throne, it’s because this is the right moment. Previously we Thais were mourning the loss of our beloved late king.”
Coup
Vajiralongkorn inherits a nation in political turmoil, with the powerful army entrenched in government for five years after staging a coup in 2014.
An election held in March has been widely seen as rigged through convoluted election laws to favour the military and its preferred candidate, Prayuth Chan-ocha, who led the coup and has headed the government since then.
He has tightened control over royal institutions and what amounts to political privileges.
He surprised the country’s ruling junta when, “to ensure his royal powers”, he requested changes to a new constitution that had already been approved in a referendum. They acquiesced.
The powers he acquired centralise royal authority in his hands and make explicit his right to intervene in government affairs, especially in times of political crisis.
After the coronation, the king will receive members of the royal family, the privy council and cabinet, among other senior officials, who will pay their respects. Afterwards he will visit the Temple of the Emerald Buddha to announce he is the royal defender of Buddhism.
The day’s events end with a ceremony of the Assumption of the Royal Residence, a symbolic palace housewarming.
On Sunday, there will be a 6.9km (4.3m) royal procession involving 343 men, some of them carrying the king through old Bangkok in an ornately decorated palanquin, allowing Thais to pay homage to their new king.
Monday will see the king greet the public from the balcony of the Grand Palace in the late afternoon and then hold a reception for the diplomatic corps. –PA