Prince Hisahito and his parents Prince Akishino and Princess Kiko pose for photos at Ochanomizu University junior high school in Tokyo. File / Koji Sasahara / AFP
Two kitchen knives have been found at the school desk of Japan's Prince Hisahito, grandson of Emperor Akihito, with security camera footage showing a suspicious man trespassing on the grounds, media reported on Saturday.
The incident comes as authorities are beefing up security ahead of the popular emperor's abdication next week after a 30-year reign.
Officials of the Tokyo junior high school found the knives on Friday at the desk used daily by the 12-year-old prince, public broadcaster NHK and other news reports said, quoting unnamed investigators.
Hisahito, who began attending the school this month, was not in the classroom when the knives are believed to have been left.
There were no reports of any injuries or damage at the school, the reports said, adding that police did not find any threatening note related to the case.
Security camera footage showed a man with a helmet trespassing on the school grounds at around noon, they said.
Police were searching for the middle-aged man who was dressed as a construction worker.
A police spokeswoman declined to comment while no immediate confirmation was available from the school.
Threats to the imperial family are relatively rare. In 1975, Akihito was almost hit by a Molotov cocktail in Okinawa, a major World War II battlefield where there was strong anti-emperor sentiment.
The incident comes as Japan is preparing for the abdication on Tuesday of Akihito, the first monarch to relinquish the throne of the world's oldest imperial family for two centuries.
Akihito's eldest son, 59-year-old Crown Prince Naruhito, will take the throne the following day in a series of ceremonies.
Hisahito is the son of Naruhito's younger brother and the last eligible male heir.
Japan's centuries-old succession would be broken if Hisahito does not have a male child as the Imperial Household Law, in place since 1947, does not allow women to ascend the Chrysanthemum Throne.
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