Image copyright AFP/ IMPERIAL HOUSEHOLD AGENCY Image caption Prince Hisahito is often guarded by police officers

Police in Japan have launched an investigation after two knives were found near the school desk of Emperor Akihito's 12-year-old grandson, local media report.

The knives were discovered on Friday in a classroom at a junior high school attended by Prince Hisahito.

Police are probing CCTV footage of a man trespassing on the school grounds.

Prince Hisahito is set to become second in line to the throne after Emperor Akihito's abdication next week.

Police believe the unidentified man caught on camera, who was dressed in blue and wearing a helmet, posed as a construction worker to access the building at Ochanomizu University.

Prince Hisahito and his classmates were in another part of the school when the knives are believed to have been planted.

While the prince is often guarded by police officers, they do not accompany him inside school classrooms, an Imperial Household Agency official told Kyodo News.

Investigative sources told Kyodo that each desk in the classroom had a piece of tape attached to it displaying the student's name, making the prince's seat identifiable.

They said the blades of the knives were painted pink.

The incident comes as Japan prepares for Emperor Akihito's abdication on Tuesday, ending his 30-year reign.

The 85-year-old is set to voluntarily step down due to health concerns in the first such abdication since 1817.

His son, Crown Prince Naruhito, will ascend the Chrysanthemum Throne the following day.

The emperor in Japan holds no political power but serves as a national symbol. The imperial family are generally popular and threats against them are relatively rare.