Police in Akiba swooped upon another man wielding a knife in Akiba, but the knife in question was a small penknife, the man absolutely innocent, and the police unable to arrest the man, as they clearly hankered to do, as the blade was under the 5cm in length permitted to be carried by Japanese law.
Police spitefully, or, depending on your point of view, overcautiously, made a show of the affair, questioning the man for 2 hours in the street, hemmed in by a dozen officers, all ready for action, without so much as a warrant. Bystanders who protested to the police at the injustice were merely shoved away.
The man harried for having the gall to carry such a tool said he used it to fix his unruly bike (it had pliers attached). He did at least apparently avoid having the police filch the blade. Source and plenty of photos at AkibaOS.