It was one of the biggest rallies in support of Japan's imperial system since the end of World War II: Some 10,300 men and women gathered at the Budokan martial arts arena to protest a proposal that would let women become empresses and pass along title to the Chrysanthemum Throne. At the end, the throng stood and raised their arms in unison while shouting, "Long live the emperor!"

What could possibly stir so much passion about monarchy in the 21st century?

The question of admitting women to the line of imperial succession, often presented outside Japan as little more than a curious anachronism, has been growing in importance for the last six months. The issue has been promoted by Japan's nationalist movement, whose influence has risen along with the controversy.

The nationalists, who offer the public a version of Japan's past that is cleansed of remorse for World War II, are now putting the issue of imperial succession — and the imperial system itself — at the heart of their appeals.

"Search all over the world, but you won't find any other family besides the Japanese imperial family that has maintained an unbroken male line for 125 generations," Takeo Hiranuma, a former minister of economy, trade and industry, said at the rally, which was organized by Nippon Kaigi, one of Japan's largest nationalist groups. "In other words, it is the precious, precious treasure of the Japanese race, as well as a world treasure."

The object of the crowd's ire was a plan by Prime Minister Junichiro Koizumi to revise the Imperial Household Law to allow a female line to hold the throne. Never mind that Mr. Koizumi has shelved the plan, after a rebellion by lawmakers in his center-right party and after an unexpected announcement last month by Emperor Akihito's second son and his wife that she was pregnant.

If the baby, due in September, is a boy, the problem will be moot for another generation, even if the emperor's first son, Crown Prince Naruhito, and his wife, Crown Princess Masako, never have their own son. The birth of a girl, however, would bring Japan back to square one. So until September, at least, there are likely to be more rallies as conservatives try to keep the issue alive.

The opposition to a female line is part of a larger nationalist movement that seeks a tougher stance against China and North Korea, presses aggressively for a revisionist history of Japan's wartime past, and pushes the myth of Japanese racial exceptionalism. Indeed, many at the rally are the same politicians, scholars and journalists who contend that the Nanking Massacre was vastly exaggerated, that Japan invaded continental Asia to liberate it and that Japan was tricked into war by the United States.

Historians trace the start of Japan's imperial system to the fourth or fifth century, though Japanese myth says the first emperor, Jimmu, a descendant of the sun goddess Amaterasu, began his reign 2,665 years ago. Political heavyweights like Mr. Hiranuma are now stating the myth as fact. In addition, the foreign minister, Taro Aso, has said that because Japanese soldiers died for the emperor, the emperor should visit the Yasukuni Shrine, the memorial to Japan's war dead and 14 Class A war criminals.

What these comments have in common is the belief that the imperial system stands at the core of Japan, defines Japan — is, in fact, Japan. To conservatives in a country that has been transformed by outside forces in everything from its laws to its social mores, the imperial system is the one institution that has remained purely Japanese.

In the imperial system, only a male relative who was a direct descendant of the imperial line could become emperor — a rule designed to keep the male bloodline pure. Eight women were allowed to reign as empresses, but only because age or marital circumstances had made them unable to bear children, which eliminated the possibility that a man outside the imperial line would father a successor, said Takeshi Hara, a professor specializing in the monarchy at Meiji Gakuin University here. An empress "had to have a pure body," he said.
Until the 20th century, concubines ensured that a male heir was born, but that practice died with the advent of modern social mores in Japan.

Nobody, perhaps, has symbolized such changes more than Crown Princess Masako, the Harvard-educated, multilingual former diplomat who married the crown prince in 1993. Back then, she represented the new Japanese woman.

Once in the palace, however, she found that only one thing was expected of her: to produce a male heir. She gave birth to a girl in 2001, and sank into a long depression. The crown prince complained in 2004 that there had been a "move to deny Masako's career and personality."

Career? Personality? This outraged conservatives who thought any Japanese woman should devote herself to bearing and raising children. Princess Masako became the bete noire of the right wing and, especially in the last six months, the target of ceaseless attacks in the popular press.

Conservatives also oppose reforms that would promote gender equality — or what the Japanese call a "gender free" society. The result is that, compared to women in other advanced countries, Japanese women have little economic or political power.

They do, however, have power over childbearing. And Japan's plummeting birthrate suggests that many women are deciding not to have children, boy or girl.