At least 10,000 people, including current and former lawmakers, have rallied in Tokyo to protest proposals that would allow a woman to assume the imperial throne.
Speakers at Tuesday's rally said efforts to alter the royal line threaten the nation's most sacred traditions. Japan has the world's oldest continuous monarchy.
The imperial family has not produced a male heir since 1965, prompting Prime Minister Junichiro Koizumi to propose a bill recommending that the emperor's first-born child - regardless of sex - ascend to the Chrysanthemum throne.
The proposal was put on hold after an announcement last month that the wife of the second son of Emperor Akihito is pregnant with her third child.
A recent poll indicated a wide majority of Japanese favor allowing a woman to inherit the imperial throne.
Some information for this report was provided by AFP, AP.