The app has since been downloaded hundreds of thousands of times as women continue to fall victim to groping on Japan's overcrowded trains.
A smartphone app developed by Japanese police is being widely downloaded by women trying to protect themselves from gropers on packed rush-hour trains.
The "Digi Police" app was originally issued by Tokyo police three years ago, but a new function designed to scare off molesters was only added a few months ago.
Since then, the app has reportedly been downloaded hundreds of thousands of times, which is rare for a government-developed app.
Japanese women on crowded trains and in other public places regularly face sexual harassment, but say they are often too afraid to call out for help due to a sense of embarrassment.
With the app, victims can press a "repel groper" icon to produce a written message saying "There is a groper here. Please help".
With another press, the message turns red and a voice repeatedly says, "Please stop!"
The app includes an alarm and can notify a designated email address when used - a feature that can also be used by children and their parents.
Users can also locate crime-prone areas and police stations on a map.
Violent crime is rare in Japan, but groping - ranging from rubbing against victims to placing a hand under their clothing - is an everyday occurrence.
It has been taken lightly as a "nuisance", leading to officials placing posters at stations and in train cars to remind passengers groping is a crime.
The Tokyo Metropolitan Police said 2620 sexual crimes were reported in 2017, including 1750 cases of groping, mostly on trains or at stations.
The #MeToo movement has been slow to catch on in largely patriarchal Japan, where speaking out often draws criticism rather than sympathy.