A Japanese man who was arrested early on Sunday inside the Imperial Palace grounds in Tokyo wearing only his underwear told police he swam across the moat to meet the emperor.
The man, in his 30s, did not seemed to have a clear political motive, said a duty officer at the Imperial Guard Headquarters.
"At the time of his arrest, around 4:34 am, he said 'I came to see the emperor. I swam'," the officer said.
His clothing was found on the side of a public road surrounding the moat, the officer said, adding that police would examine whether he is mentally fit to stand in criminal proceedings.
Light but cold rain fell in Tokyo early on Sunday, with temperatures around 4.0 degrees Celsius.