A sticker created by Saitama Prefectural Police to deter gropers on trains has proved so popular among women that stocks have almost run out.

The police began distributing the stickers for free to women and girls in mid-January. Measuring 2.5 cm, the round, dual-layer sticker bears the message "Don't touch!" in Japanese and is intended to be stuck on something a commuter will be carrying, such as a mobile phone or a holder for a transit pass.

If the person bearing the sticker is groped, she can show it to the culprit "as a warning," said Riichi Oto, deputy chief of the prefecture's railway police.