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Naruhito became emperor in May this year in a brief, tradition-filled ceremony but Tuesday's "Sokui no Rei" was a more elaborate ritual at the royal palace in which he officially announced his change in status to the world.
"I swear that I will act according to the constitution and fulfil my responsibility as the symbol of the state and of the unity of the people," the 59-year-old declared, his voice slightly hoarse, in front of about 2000 participants, including Britain's Prince Charles.
The first Japanese emperor born after World War 2, Naruhito acceded to the throne when his father, Akihito, became the first Japanese monarch to abdicate in two centuries after worrying that advancing age might make it hard to perform official duties.
"As he is young and energetic with outstanding leadership, I hope he'll support the people of Japan, which has faced continuous disasters and typhoons," said Tomoko Shirakawa (51), who was among small groups of umbrella-clutching supporters outside the palace.
The long-planned celebrations were tempered by Typhoon Hagibis, which killed at least 80 people when it tore through Japan 10 days ago, and pouring rain on Tuesday.
A public parade was postponed until next month to allow the government to devote its attention to the typhoon clean-up, while Tuesday's inclement weather forced the palace to scale back the number of courtiers in ancient robes taking part in the courtyard ceremony although the skies cleared as it began.
Naruhito began the day's ceremonies by reporting his enthronement to his imperial ancestors at one of three shrines on the palace grounds, dressed in a black headdress and pure white robes with a long train borne by an attendant.
He was followed later by Empress Masako (55) his Harvard-educated wife, dressed in 12-layered white robes and attended by two women in violet robes to arrange her train.
For the main ceremony in the Matsu-no-Ma, or Hall of Pine, the most prestigious room in the palace, Naruhito wore a traditional burnt-orange robe and headdress, as his father did nearly three decades ago.
He declared his enthronement from the "Takamikura" - a 6.5-metre high pavilion that weighs about 8 tonnes - with an ancient sword and a jewel, two of the so-called Three Sacred Treasures, placed beside him.
Together with a mirror called Yata-no-Kagami, which is kept at the Ise Grand Shrine, the holiest site in Japan's Shinto religion, the three treasures comprise the regalia that symbolises the legitimacy of the emperor.
Masako, wearing 12-layered robes and an elaborate, upswept hairstyle, also took part, her throne smaller.
Prime Minister Shinzo Abe delivered a congratulatory speech before guests including Prince Charles, who with then-wife Princess Diana attended Akihito's enthronement, as well as US Transport Secretary Elaine Chao and Myanmar civilian leader Aung San Suu Kyi.
Abe then led the assembled dignitaries in giving three "banzai" cheers for the Emperor.
IMPERIAL FAMILY FUTURE
A court banquet was to be held on Tuesday evening, before Naruhito and Masako host a tea party for foreign royalty on Wednesday afternoon.
While the public parade was postponed until November 10, the NHK national broadcaster said there were 26,000 police providing security on Tuesday.
Naruhito is unusual among recent Japanese emperors since his only child, 17-year-old Aiko, is female and as such cannot inherit the throne. The future of the imperial family for coming generations rests instead on the shoulders of his nephew, 13-year-old Hisahito, who is second in line for the throne after his father, Prince Akishino.
Naruhito's grandfather, Hirohito, in whose name Japanese troops fought World War Two, was treated as a god but renounced his divine status after Japan's defeat in 1945. Emperors now have no political authority.
Though many Japanese welcomed the enthronement ceremony, some shrugged it off as a nuisance.
"There is no need for such an elaborate ceremony. Traffic has been restricted and it is causing inconvenience for ordinary people," said Yoshikazu Arai (74), a retired surgeon.
"The emperor is necessary now as a symbol of the people, but at some point, the emperor will no longer be necessary. Things will be just fine without an emperor."