Tokyo may have one of the cleanest and most convenient commuter train systems in the world, but it has its pests.

In the first six months of this year police recorded more than 700 assaults by gropers on Tokyo's notoriously crowded trains.

This week police have launched Groping Prevention Week and investigators are also targeting websites which provide advice on the best train lines to go to to fondle women.

Every morning across Tokyo, millions of commuters take a deep breath and join the jam on the city's trains.

It is not uncommon at the larger stations like Shinjuku to see station attendants actually pushing people into already overflowing carriages.

This is the sort of environment where the gropers thrive.

Last year police recorded more than 6,000 cases involving women being fondled or surreptitiously photographed.

An anti-groping television advertisement shows more than 100 high school girls at Tokyo's Ikebukuro station holding up a banner that reads: "Groping is contemptible. We don't forgive it!"

"Please help exterminate the gropers," they shout.

Eight years ago, in an effort to exclude the gropers, Tokyo began running women-only carriages during rush hour.

Now those caught molesting women face up to six months in jail and a fine of $6,000.

But that has not stopped hundreds of men preying on female commuters.

This week at a train station in Tokyo, a policewoman held classes to demonstrate how to deal with a groper.

"I often carry a bag and umbrella so I think I can swing it like a weapon if I need to," a young woman said.

A policewoman says the techniques used by train gropers are becoming more clever and wicked.

It is believed thousands of women put up with the abuse and say nothing.

Surveys suggest two-thirds of young women in Tokyo have been molested on trains and prosecutors have found it difficult to get convictions because the men they're after are often cunning and hunt in packs.

Police say there are now more than 100 websites devoted to train groping.