TOKYO, NOV. 12 (MONDAY) -- Clad in ancient court robes and a tall, plumed crown, Japan's Emperor Akihito today observed a lavish ritual performed by his ancestors for at least a dozen centuries as he formally acceded to the Chrysanthemum Throne, extending the oldest monarchal dynasty on Earth.

A warm afternoon breeze carried the congratulatory shouts of "Banzai!" over the graceful fluted roofs of the Imperial Palace here. A brilliant autumn sun glistened on the multicolored banners and golden court regalia as the 56-year-old sovereign proclaimed his ascendancy to his nation and the world.

Accompanied by his wife, Empress Michiko, and attended by squadrons of courtiers in exotic gowns who carried five-foot-tall flags and played ceremonial gongs and drums, the emperor declared his allegiance to Japan's post-war constitution, which makes him a "symbol of the state" with no governing authority.

The emperor made his brief pronouncement from his Takamikura, or "high honored place," a spectacular purple-and-laquerware stage with an ornate 20-foot-high octagonal canopy topped with a golden statue of a phoenix.

The empress, wearing a five-layer red, green, blue and silver kimono that weighs more than she does, stood perfectly still on her own ornate stage to Akihito's left.

On hand to observe the antique rite were representatives from 158 countries, including Vice President Quayle, and Prince Charles and Princess Diana of Britain. The foreign guests were excused from joining in the thrice-repeated shout of "Banzai!"

"Banzai!" means "10,000 years" and is the Japanese equivalent of "Viva!" or "Long live the king!" But the word became a controversial symbol of Japan's aggressive past because of its widespread use by Japanese soldiers in World War II.

After that war, the new constitution imposed on Japan by the United States completely changed the status of the emperor, and Akihito -- whose name means "shining pinnacle of virtue" -- is the first sovereign to take the throne since those changes were made.

While his father, the late Emperor Hirohito, was a figure of awe for whom millions of Japanese proudly laid down their lives, Akihito's position is constitutionally restricted to symbolic status.

As a sign of the emperor's dimished status, Prime Minister Toshiki Kaifu offered congratulations on behalf of the nation while standing in the same room and at the same level as Akihito.

This was a sharp break with history for a nation where strict hierarchical distinctions have long been enforced by making lesser people stand or sit lower than the mighty.

The elaborate spectacle surrounding the enthronment has spurred tepid enthusiasm at best among the Japanese people. In a recent survey, only 13 percent said they were seriously interested in this rare imperial event.

But because a relatively tiny number of Japanese are violently opposed to the imperial system, security precautions for the ceremonies have been ubiquitous and relentless.

About 37,000 police are on the streets of downtown Tokyo; coin lockers, vending machines, and even public trash cans have been sealed shut; a police blimp hovered overhead warning citizens to report anything or anyone suspicious.

A dozen or so incendiary bombs exploded around Tokyo this morning near military bases and Shinto shrines. None did serious damage, police said.

In addition, about 50,000 leftist trade unionists staged a rally in a central Tokyo park Sunday to protest the ceremonies, which are costing Japanese taxpayers about $100 million, but the demonstration was peaceful, Reuter reported.

The ceremonies began early Monday when Akihito visited three Shinto shrines to report to the sun goddess and to his 100-plus imperial ancestors that he was about to take the throne.

The day's principal ritual was a half-hour ceremony called the Sokui no rei, or "enthronement ceremony," although Akihito never actually took a seat on the throne. The Takamikura, representing a mountain where the sun god is said to have placed her grandson to begin the imperial family in 600 B.C., will now be dismantled and packed away, not to be used again until the next emperor's enthronement.

Akihito has actually been Japan's emperor since the moment of his father's death on Jan. 7, 1989, and that day marked the official start of his reign, which is known as the Heisei, or "Achieving Peace" era.

But for reasons that reach back to early chapters of Japanese history, he could not officially complete the ceremonies of accession until this month. The rules governing this enthronment are based closely on those set forth in the Taiho Code, promulagated in 1701.

As part of the Taiho enthronement ceremonies -- a series of events that continue, off and on, for about three weeks -- Akihito later this month will dedicate part of the rice harvest to the gods. But because the code dictates that he can only dedicate rice planted after the one-year mourning period for his father has ended, he had to wait until the 1990 rice crop was harvested this fall.

By court tradition, Akihito is the 125th in an unbroken succession of emperors. But historians dismiss the first 14 as mythical figures, and the line was broken once in the 6th century.