Divers have scoured moats around the Imperial Palace and police dogs sniffed in bushes as authorities ramped up security for the start of Japan's new "Reiwa" imperial era.
The new era begins on Wednesday when Crown Prince Naruhito ascends the Chrysanthemum Throne a day after the abdication of his father, Emperor Akihito, brings to an end the 31-year Heisei era.
Police stepped up security at the Imperial Palace, a 115 hectare site that is home to the emperor and empress in the heart of Tokyo.
Officers poked at bushes with steel sticks to check for hidden objects, while dogs sniffed nearby. Divers checked the murky water of a dozen moats that surround the palace.
Public access to the plaza in front of the palace will be restricted during the abdication and ascension ceremonies.
Security will be tight at Naruhito's residence in the Akasaka royal estate, a police official said.
"We will be enhancing security to prevent illegal behaviour, including terrorism acts, from happening near the palace and the Akasaka Imperial property as well as streets accessing them," said the official who declined to be identified because he was not authorised to speak to media.
Kyodo News said several thousand police officers will provide security for the ceremonies and a public appearance by the new emperor on Thursday when huge crowds of well-wishers are expected.
Police on Monday declined to comment on media reports that two knives were found on the classroom desk of Prince Hisahito, the 12-year-old grandson of Emperor Akihito.
Security camera footage showed a man disguised as a construction worker enter the grounds of the junior high school in Tokyo, Kyodo News said, citing police.
