Emperor Naruhito succeeded to the Chrysanthemum Throne on Wednesday and pledged to stand with his people following the example set by his father, who abdicated the previous day as the first Japanese monarch to do so in 202 years.
The 59-year-old emperor, the first to be born after World War II and to have studied overseas, also vowed in an enthronement ceremony to fulfill the symbolic, nonpolitical role in accordance with the postwar Constitution.
"In acceding to the throne, I swear that I will reflect deeply on the course followed by his majesty the emperor emeritus (Akihito) and bear in mind the path trodden by past emperors, and will devote myself to self-improvement," he said in his first speech after enthronement.
"I also swear that I will act according to the Constitution and fulfill my responsibility as the symbol of the state and of the unity of the people of Japan, while always turning my thoughts to the people and standing with them," he said during the "Sokui go Choken no gi" rite.
He was flanked by Empress Masako, 55, a Harvard- and Oxford-educated former diplomat.
The ceremony at the "Matsu no Ma" stateroom in the Imperial Palace was attended by roughly 300 people, including the heads of the government, legislature and judiciary.
Other imperial family members, including the emperor's brother Crown Prince Fumihito, 53, his wife Crown Princess Kiko, 52, and their two daughters, were also present.
Following the emperor's speech, Prime Minister Shinzo Abe congratulated him on his enthronement on behalf of the people, and promised to create a "bright future" that is peaceful and full of hope by respecting him as a symbol of the state.
In the preceding ceremony known as "Kenji to Shokei no gi," the emperor, dressed in a black tailcoat and wearing a number of decorations marking his status, inherited the imperial regalia as proof of his ascension to the throne.
The regalia, called "Sanshu no Jingi," consist of the sacred mirror, sword and jewel. The original mirror is kept at Ise Jingu, a Shinto shrine in Mie Prefecture, central Japan, and the sword at Atsuta Jingu in Nagoya, Aichi Prefecture.
In the ritual, the jewel and a replica of the sword were passed to the new monarch together with the state and privy seals. At the same time, an aide to the emperor visited a shrine inside the Imperial Palace where a replica mirror is kept.
Both ceremonies lasted less than 10 minutes.
In addition to government officials, only adult men from the imperial family -- the crown prince and his uncle Prince Hitachi, 83 -- attended the inheritance ceremony for the regalia and seals, following the precedent set at former Emperor Akihito's enthronement in 1989.
The now-retired emperor did not attend Wednesday's ceremonies.
Female members of the imperial family were barred from the inheritance ceremony because they are not allowed to succeed to the throne under the Imperial House Law.
However, a kimono-clad Satsuki Katayama, minister in charge of regional revitalization, became the first woman to attend this ceremony since the late 19th century.
With the enthronement of Emperor Naruhito, the crown prince's son Prince Hisahito, 12, became second in line to the throne, followed by Prince Hitachi.
The Japanese imperial family has been overshadowed by its contraction, with female members losing their royal status upon marriage to commoners. Of the 18 existing members of the family, 13 are women and Prince Hisahito is the only male in his generation.
In the face of the growing risk of the imperial line's demise, the government is expected to start considering measures to sustain the family more stably possibly this fall.
Following the two rituals, the new emperor appointed -- in another ceremony -- former diplomat Nobutake Odano as grand chamberlain and the previous grand chamberlain, Chikao Kawai, as the retired emperor's top aide official.
Emperor Naruhito later left the Imperial Palace via the main gate, which is only used by emperors, carrying with him the inherited jewel and sword to his nearby Akasaka Estate residence.
In the afternoon, the imperial couple visited former Emperor Akihito and former Empress Michiko at their residence inside the Imperial Palace. The new imperil couple were later congratulated by imperial family members, Imperial Household Agency officials and others.
People across Japan celebrated the emperor's ascension and start of the Reiwa Era at a plethora of celebratory events.
The public will have to wait until Saturday to greet the new emperor and empress, who will make their first public appearances at the Imperial Palace during the Golden Week holiday through Monday. The annual holiday has been extended this year to 10 days to celebrate the imperial succession.
Emperor Akihito stepped down midnight Tuesday, bringing an end to the Heisei Era spanning his 30-year reign. "I have performed my duties as the emperor with a deep sense of trust in and respect for the people, and I consider myself most fortunate to have been able to do so," the emperor said in his last speech Tuesday.
In modern Japan, imperial era names, or "gengo," are widely used in Japanese calendars, on coins and on official documents, and the new name was announced on April 1 to facilitate changes to these. Reiwa is translated by the government as "beautiful harmony."
A series of ceremonies and events for the imperial succession are scheduled in coming months, including the "Sokuirei Seiden no gi," to proclaim the enthronement of the emperor in the palace's state hall on Oct. 22.
After this ceremony, the new emperor and empress will parade in an open-top limousine through Tokyo, and take part in banquets the same month, attended by more than 2,000 guests.
Daijosai, or the great thanksgiving ceremony, in November always follows an emperor's accession to the throne. The emperor will make offerings to ancestral deities and pray for the peace and prosperity of Japan and its people.
The numerous ceremonies will end with the emperor's visits to the mausoleums of past emperors and Ise Jingu, possibly by the end of the year.
As imperial successions usually happened upon the death of an emperor, the successor had to take part in mourning and funeral events simultaneously with ceremonies for his enthronement.
Former Emperor Akihito, enthroned aged 55 on Jan. 8, 1989, a day after his father Emperor Hirohito died, indicated his wish to abdicate in an August 2016 video address, saying he might not be able to fulfill his duties due to his advanced age. In 2017, Japan enacted one-off legislation to enable him to step down.