This app was mentioned in 3 comments, with an average of 2.00 upvotes
I use this app.
You'll need a video of whatever you are reporting on which the number plate is visible. So, if you want to report a scooter riding on the side walk, you have to get pretty close. In addition, the police requires a time stamp on the video, similar to what dashcam videos look like. Most camera apps can't do that, so I use the premium version of this app to add a time stamp and the GPS coordinates.
As long as the vehicle's number plate is visible, the police usually responds within a few days and fines the driver. I guess I've caused several million Won in fines that way by now, mostly for the drivers of scooters who ride on side walks, use pedestrian crossing or go over red lights. I also got several bus drivers fined.
Use 국민신문고 or this app.
https://play.google.com/store/apps/details?id=kr.go.police.cyber112
You can report them with photo or video. If you don't have any proof, you could provide more details like bus number, plate number, exact time and location so they could identify the driver and check the dash cam
I think it's this app, but I havent tried it yet:
스마트국민제보 https://play.google.com/store/apps/details?id=kr.go.police.cyber112