In OTL, Japanese police carried katana up until 1945 when American occupation forces, who considered katana to be a badge of Japan's militaristic past, insisted that Japanese police replace katana with American style automatic pistols.
Prior to the Meiji "Restoration", police were simply local low-ranking samurai who served as judge, jury, and police all in one. They carried swords simply as a badge of their samurai rank. In 1872, a new police force was set up, modeled after the French and Prussian examples. These new police were sometimes called upon to act as ad-hoc militia in the early days, notably in the 1877 Satsuma Rebellion. As such, they used the same weapons that the Army did:
Rifles:
- Type 38 Rifle
- Type 44 Cavalry Rifle
- Type 97 Light Machine Gun
Pistols & Revolvers:
- Type 26 9 mm Pistol
- Type 14 8 mm Nambu Pistol
- Type 94 8 mm Pistol
It's true that police officers had swords, but these were merely part of their dress uniform. They were worn on parade, not on the beat, and they simply weren't meant to be true, fuctional weapons. I'm sure the police swords were used sometimes, but this would have been exceedingly rare. An average beat cop would carry no weapon at all save a baton. If it became necessary to carry a weapon for some special dangerous duty, they would skip the swords and go straight for a pistol or carbine.
Gerdarme would have had swords, but that was because of their nature as military police, not just because they were police.
In the postwar period, the US gave quite a few S&W revolvers (not automatics) to the newly reformed Japanese police. The Japanese kept using those, plus some Taurus models, and some ddomestic. 38 spcl revolvers. The US was the same way. The vast majority of US police departments continued using revolvers up until the 1980s, and only switched over to automatics with the development of the so-called "wonder nines" in that decade. I'm not sure what Japanese police carry now. I imagine they, too, have switched over to some polymer-framed auto by now, but during my time in Japan I'm not sure I've ever noticed a policeman carrying a weapon. The average beat cop still doesn't necessarily carry one, much like in the UK. But then, I wasn't really looking. I'm sure I have seen one and just not noticed.
Edit: Apparently
this is the main Japanese police reolver.
I believe it is still in use. Plain clothes units and others needing deep concealment have switched over to
this instead.