George Tupou V has been crowned king of Tonga in the capital Nuku'alofa, marking the South Pacific state's first coronation in more than 40 years.
He was anointed with oil and had a gold crown placed on his head in the Christian ceremony, performed in a church before 1,000 guests.
The new monarch was called upon to rule "wisely, justly and truly".
Thousands of people had lined the route to the church and cheered when the new monarch emerged.
The occasion marked "the opening of a new era in our journey as a nation - an era of political and economic reform and increased prosperity", Prime Minister Fred Sevele told guests, according to AP news agency.
The new king's father, the late King Taufa'ahau Tupou IV, ascended to the throne in 1967 and died in 2006.
Sitting on a golden throne on Friday in the capital's Centenary Free Wesleyan Church, George Tupou V was "anointed, blessed and consecrated" by the Archbishop of Polynesia, Jabez Bryce.
The king wore silk knee breeches, a medal-decked jacket and a maroon-coloured cape trimmed with white ermine fur.
His three-metre-long (10-foot) train was carried by child pages.
A 21-cannon salute and the tolling of church bells marked the coronation.
The ceremony was followed by an extravagant lunch, and then traditional dancing, a fireworks display and an open-air royal charity concert.
There were three separate coronation balls - one for guests listed as "VVIPs" (very, very important persons), one for "VIPs" and a third for regular guests, reported AP.
Royal guests included Japanese Crown Prince Naruhito, Thai Princess Maha Chakri Sirindhorn and the UK's Duke and Duchess of Gloucester.
Also in attendance was New Zealand Prime Minister Helen Clark, who said: "If you went for no other reason, you would go for the singing," reported AFP news agency.
Leaders have defended the lavish nature of the festivities, which have cost some 5.7m Tongan dollars (US$2.5m) in a country where poverty is widespread.
The new king will rule over a semi-feudal political system where he and nobles decide the make-up of the cabinet and parliament.
But he has said he supports reforms, scheduled for 2010, in which most seats in the country's parliament will become popularly elected.
The promised reforms follow destructive riots in Nuku'alofa in 2006.