Almost two decades after the introduction of women-only train carriages, female commuters in Japan are turning to technology to tackle molesters on packed rush-hour trains.

The Digi Police app enables victims of groping to activate a voice shouting “Stop it!” at ear-piercing volume or bring up a full-screen message reading, “There is a molester. Please help” that they can show to other passengers.

Digi Police has been downloaded more than 237,000 times since it was introduced three years ago – an “unusually high figure” for a public-service app, according to police.

“Thanks to its popularity, the number [of downloads] is increasing by about 10,000 every month,” said police official Keiko Toyamine.

Toyamine said victims were often reluctant to call for help, but the app’s SOS message allows them to alert other passengers while staying silent.

The Tokyo metropolitan police department recorded almost 900 cases of groping and other forms of harassment on trains and subways in the capital in 2017.

Toyamine, however, said the figure was just the “tip of the iceberg”, since many victims are reluctant to come forward.

Groping is a perennial problem on Japanese public transport, particularly on packed rush-hour trains, when identifying the culprit can be difficult.

Offenders face up to six months in prison or fines of up to 500,000 yen (£3,600). The potential sentence rises to 10 years if violence or threats are involved.

The app was originally intended to warn vulnerable older people about financial scams and to provide safety information to parents and children.

The function to alert passengers to molesters was added a few months after the app’s launch, with interest soaring late last year after a female pop star was assaulted at the entrance to her home by two obsessive male fans.

Reina Oishi, a 21-year-old university student in Tokyo, said: “I want to download the app as I have been groped so many times.”

Women-only train carriages have become a fixture in Japanese public transport since they were introduced by a private railway firm in Tokyo in 2001.

Women can now use the carriages – most of which are available during the busy morning and evening commutes – on 87 lines operated by 32 rail firms, according to the Mainichi Shimbun newspaper.

Agence France-Presse contributed to this report.