Former Emperor Akihito and Empress Michiko stepped down on Tuesday after three decades as the nation's top royals in a brief, simple ceremony, with Akihito thanking the people of Japan and saying he prayed for peace.
Naruhito (59) technically succeeded his father just as Tuesday became Wednesday but his ascension to the Chrysanthemum Throne was formalised in a mid-morning ceremony, the first part of which his wife and other royal women were not allowed to attend.
Naruhito, the first emperor born after World War 2 and the first to be raised solely by his parents, expressed gratitude for his parents' work over the past three decades on Wednesday and said he felt solemn at the thought of the burden he is taking on.
"I sincerely hope for the happiness of the people and further progress of the country, and for world peace."
In the first stage of the accession ceremony, imperial chamberlains carried state and privy seals into the hall along with two of Japan's "Three Sacred Treasures" - a sword and a jewel - which together with a mirror are symbols of the throne. They are said to originate in ancient mythology.
He was flanked by his brother and heir, Crown Prince Akishino, during the simple, brief ceremony.
His wife, Empress Masako, was not in the room in accordance with custom barring female royals, but for the first time a woman did watch - Satsuki Katayama, who was taking part as a member of Prime Minister Shinzo Abe's cabinet.
Masako, wearing a floor-length white dress and a tiara, entered the room for the second part of the ceremony with the other adult royal women.
Naruhito's formal enthronement will take place at a more elaborate ceremony in October attended by dignitaries from Japan and around the world.
Given the backgrounds of Naruhito and his wife, Masako, a 55-year-old former diplomat - which include extended experience studying and living overseas - hopes are high that they may be more international in their outlook and closer to the lives of many Japanese.
"The curtain has gone up on a new era that will be filled with hope," said Hiroshi Takahashi (78) outside his 'wagashi' traditional Japanese sweets shop.
"The new emperor has an admirable personality and I hope he creates a fine royal household that expresses his own personality," he said.
Though Masako struggled with a stress-related illness described by palace officials as an "adjustment disorder" that kept her largely out of the public eye for years, her public appearances have recently increased.
Their only child is a daughter, Aiko (17), who cannot inherit the throne.
Naruhito's heirs are his younger brother, Prince Akishino, followed by Akishino's 12-year-old son, Hisahito.
FESTIVE TIME
The last imperial succession in 1989 took place during mourning for Akihito's father, Hirohito.
This time the mood is more festive. Japan has been draped in banners welcoming Reiwa - the name of the new era for Naruhito's reign - during an unprecedented 10-day holiday.
Countdown events were held on Tuesday night in clubs throughout the nation, with people cheering as the clocked ticked down to midnight and fireworks shooting into the sky in some areas.
On Wednesday morning, electric signs in Tokyo's subway system bore messages "congratulating the emperor on his accession" and workers in a downtown Tokyo electronics store wore red shirts saying "Happy New Era Reiwa."
Though the royal family is broadly popular, opponents of the imperial system clashed with right-wingers at a demonstration on Tuesday, and the two groups had to be separated by police.
Japanese media said two people were arrested.
