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 function to scare off molesters was only added a few months ago. Since then, the app has reportedly been downloaded hundreds of thousands of times — unusual for a government-developed mobile application.

Women in crowded trains and other public places in Japan often face sexual harassment but are typically too afraid to call out for help.

With the app, victims can press a "repel groper" icon to produce a message saying "There is a groper here. Please help." With another press, the message turns red and a voice repeatedly says, "Please stop!"