Japanese media reports say police are investigating a knife that was placed on the school desk of Emperor Akihito's grandson.

The incident involving 12-year-old Hisahito, who is in succession line for the Chrysanthemum Throne, comes as Japan prepares for ceremonies marking Akihito's abdication on April 30.

Kyodo News says police are investigating security camera footage. The Metropolitan Police Department has declined comment and Ochanomizu University Junior High School did not answer calls.

Akihito's son, Crown Prince Naruhito, is set to become emperor May 1. Hisahito is Naruhito's nephew.

The emperor, revered as a god during World War II, now holds no political power and serves as national symbol.

Akihito and his family are generally popular but have been targeted in attacks in the past.