It is one of the safest cities in the world, but for teenage girls and young women, travelling on Tokyo's vast train and subway system comes with the ever-present risk of sexual harassment from fellow commuters.

But this week the Japanese capital's police launched a crackdown on the scourge of groping. In the first reported case, Tsutomu Yamane, 30, was arrested with minutes of allegedly groping a 15-year-old girl on Monday morning.

Undercover police officers have been dispatched to notorious train lines across Tokyo in a new offensive against chikan, a catch-all term that covers groping, sexual rubbing and surreptitious mobile phone photography.

The introduction almost a decade ago of women-only carriages during the morning and evening rush hours failed to put an end to Japan's chikan plague, despite warnings that convicted gropers face up to six months in prison and a fine of up to 500,000 yen (£3,300).

Police in Tokyo make between 1,500 and 1,800 arrests a year, and have recorded more than 700 cases in the first six months of this year. More than 50% of victims are women in their 20s, and about a third are teenage girls. According to one survey, nearly two-thirds of young women say they have been touched inappropriately in train or underground carriages.

Kimiko Kitagawa, a 31-year-old business consultant, joined the long list of chikan victims as she stepped off a subway train near her office in central Tokyo earlier this year. "I felt a hand grab my backside, but when I turned around there were several men rushing to get past," she said. "I had no way of knowing who had touched me."

Confusion and embarrassment mean the number of groping incidents is suspected of being much higher than official reports suggest. "Many victims are reluctant to come forward, as they feel too embarrassed," a police spokesman told the Yomiuri newspaper. "We are going to concentrate on the most serious cases."

The crackdown came in response to a recent jump in cases involving groups of men who use online chat rooms to arrange where and when to target women. At least 100 websites list prime groping locations, offering hints on how to fondle undetected and, if that does not work, evade arrest.

One site advises would-be gropers to select carriages with doors that open near platform escalators or staircases – perfect escape routes.

Several men arrested in recent months have admitted being encouraged by the websites and emboldened by the prospect of working as part of a group. A typical tactic is to position two men in front and behind a victim, while as many as six other men block the view of other passengers.

The police are targeting nine railway lines in Tokyo where groping has become endemic, especially the Saikyo line, which takes commuters between Ikebukuro and Akabane in the capital's suburbs.

Attempts to prosecute gropers, however, have been frustrated by cases in which women, often with the help of male accomplices, make have made false accusations in the hope of securing huge out-of-court settlements.

Men who are apprehended are usually found guilty. The 2007 film, I Just Didn't Do It, was based on the true story of a young man accused of groping who cleared his name after a five-year battle. In another high-profile case, the supreme court last year acquitted a 63-year-old professor of molesting a teenage girl in 2006. He had earlier been sentenced to 22 months in prison.