By Roland Buerk
BBC News, Tokyo
Tokyo's trains are notoriously crowded at rush hour
Tokyo police have begun a week-long crackdown against the twice-daily scourge of gropers on commuter trains.
Undercover teams have been deployed on some lines in a bid to catch molesters in the act on crowded trains.
Last year more than 6,000 people were arrested on suspicion of groping or taking unsolicited photographs.
According to one survey, nearly two-thirds of young women have been groped on public transport. Some train lines have introduced women-only carriages.
Tokyo police have begun what is being described as a "groping prevention week".
There are conspicuous extra police patrols in stations handing out leaflets, and undercover teams have been deployed on trains to try to catch men in the act.
Gropers can be imprisoned for up to seven years in Japan.
Local reports say the police are particularly concerned that gropers are using the internet to co-ordinate their activities and form gangs.
Several suspects arrested in recent months are said to have told officers they had targeted particular train lines because of recommendations they had read on websites.