He will hand the Chrysanthemum Throne to his son and usher in a new imperial era.

In a set of solemn ceremonies, Emperor Akihito will abdicate in favor of 59-year-old Crown Prince Naruhito, also kicking off the new imperial "Reiwa" era - meaning "beautiful harmony" - that will last throughout the new monarch's reign.

The ritual-bound process began Tuesday morning with the emperor, dressed in expansive golden-brown robes and a towering black headpiece, "reporting" his abdication to his ancestors and the gods at several "sanctuaries" at the palace.

But the main event will be at precisely 5:00 p.m. local time (0800 GMT), when the 85-year-old will formally step down in a 10-minute ceremony in the "Matsu-no-Ma" ("Room of Pine"), considered the most elegant hall in the sumptuous Imperial Palace.

The ritual will be conducted in the presence of the imperial regalia - an ancient sword and jewel - considered crucial evidence of an emperor's legitimacy.

However, Naruhito will not become emperor of Japan until the stroke of midnight and he will "inherit" the regalia at a second ceremony Wednesday at 10:30 a.m. before making his first official public remarks shortly afterwards.

Tomorrow's ceremony was expected to be attended by just one woman -- the sole female member of Prime Minister Shinzo Abe's cabinet. Female royals are prohibited from participating.

The ceremonies will all take place behind closed doors, but small crowds had gathered outside the Imperial Palace in Tokyo on Tuesday morning, despite clouds and rain.

"I'd like to thank the emperor for his hard work," said 76-year-old Hironari Uemara, visiting Tokyo from Okayama in western Japan.

His wife said she would miss Akihito and the outgoing imperial Heisei era.

"I feel like crying," she told AFP.

Popular touch

The popular Akihito stunned Japan when he announced in 2016 that he wanted to give up the Chrysanthemum Throne, citing his age and health problems - he has been treated for prostate cancer and has also undergone heart surgery.

There have been abdications in Japan's long imperial history, which has mythological origins and stretches back more than two millennia, but the last one was more than two centuries ago.

A more lavish and public enthronement ceremony attended by world leaders will take place on October 22.

Akihito has sought to modernise the imperial family in Japan, which has a sensitive position given the role his father Hirohito played in the country's militaristic past.

He and his wife Empress Michiko won plaudits for a popular touch, notably comforting people affected by the 2011 earthquake, tsunami and nuclear meltdown that devastated whole swathes of east Japan and killed thousands.

Images of the couple kneeling and bowing to those in temporary shelters gave some heart to the stricken nation and Akihito took the rare step of giving a televised address to reassure his people.

Like his father, Crown Prince Naruhito is seen as a modern royal and has previously issued mild criticism of the sometimes stifling lifestyle imposed on royals, particularly as his wife Masako has struggled to adapt to imperial life and has long battled stress-induced illness.

Succession fears

The new emperor inherits a country very different to when it last crowned a new emperor.

In 1989, when Akihito ascended the throne, Japan ruled the world economically in the middle of a technology-fuelled boom that caused soaring land prices and sparked wild cost comparisons: the Imperial Palace grounds were worth more than all of Canada.

Now, Japan's population is in decline and it is on course to become the world's first "ultra-aged" society, with 28 percent of people over 65.

The boom gave way to a "lost decade" of tepid economic growth and deflation from which Japan has not fully recovered.

The abdication has also reignited concerns about a potential succession crisis. There are no more eligible male heirs after the 12-year-old son of Naruhito's younger brother Akishino.

Japan's centuries-old succession would be broken if that son, Hisahito, does not have a male child. The idea of letting women ascend the throne is popular with Japanese, but vehemently opposed by traditionalists.

The historic abdication has resulted in an unprecedented 10-day holiday for the famously hard-working Japanese, as special days off to mark the new emperor combine with the traditional "Golden Week" celebrations in May.

As the holiday kicked off over the weekend, bullet trains and airports were packed, but the exodus left the capital's commuter trains unusually empty.