About 40 Japanese lawmakers have joined a rally in Tokyo to protest against government plans to allow women to ascend to the throne.
The daughter of the emperor's eldest son (left) could succeed the throne
Former Trade Minister Takeo Hiranuma told supporters that the move could dilute the imperial line.
A bill to change the Imperial Household Law's succession rules is expected to be presented to parliament this year.
The imperial family is facing a crisis, having produced no boys since 1965, but some conservatives oppose any change.
The Imperial Household's main succession hopes lie with four-year-old Princess Aiko, the daughter of Crown Prince Naruhito and his wife Masako.
This, Mr Hiranuma argued, could be dangerous.
Currently only males can ascend the Japanese throne
Emperor Akihito has two sons, Naruhito and Akishino
If Naruhito died without a male heir, his brother, Akishino, succeeds
But he has no sons either
Their sister, Princess Sayako, is marrying a commoner so her children cannot ascend throne
"If Aiko becomes the reigning empress, and gets involved with a blue-eyed foreigner while studying abroad and marries him, their child may be the emperor," he told the rally at a Tokyo hall.
"We should get united and prevent the Cabinet from submitting legislation to parliament," he said.
"We'll do our best to preserve the authentic tradition and culture and protect our nation."
Some 1,200 people attended the rally, according to organisers.
Another prominent critic of the plans, a cousin of Emperor Akihito, repeated his misgivings on Wednesday.
"It would be troublesome to make a decision by going down to the level of recognizing that 'Princess Aiko is adorable' or 'Princess Masako would be freed from the pressure to produce an heir,'" Prince Tomohito told Wednesday's Sankei Shimbun.
Analysts believe the pressure on Princess Masako to bear a male heir contributed to stress-related illnesses which stopped her fulfilling many official duties.
Prince Tomohito has previously suggested Emperor Akihito's eldest son Naruhito take concubines to ensure a male heir, or that families who lost their royal titles after World War II be reinstated.
Japan has had female monarchs before - between the 6th and 18th centuries - but all have reigned in emergency circumstances and none had children who then ascended the throne.