In poor Kenyan communities the rates of sexual assault are staggeringly high — but also hard to quantify.

That is because most are never reported.

But a group of grandmothers has had enough and they are fighting back — literally.

Korogosho is one of the many informal settlements in Nairobi. It is a slum.

It is there that a group of grandmothers is using elements from wrestling and karate to protect themselves and each other as part of a program that tries to prevent sexual assault.

Ferocious grandmother, 74-year-old Beatrice Nyariara, is in the middle of training.

"It is very unsafe because before we started this group of old women like me, a lot of old women used to be raped by young men. They take drugs, alcohol and when it enters their head, they can't differentiate between their elders and young girls," Ms Nyariara said.

"We found someone to train us. We started doing karate. As we continued, young men realised we won't be hurt and started backing."

Most of the women in this group have either been sexually assaulted personally, or they know someone who has.

The woman learn elements of wrestling and karate for self-defence. ( ABC News: Martin Cuddihy )

Kenya police estimate that 85 per cent of sexual crimes are not reported.

Jane Waithegeni was gang-raped more than 10 years ago. She was abducted from a public bus.

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"They removed my shirt and blindfolded me. I was taken to a second car, then to a third car. I was held for three days and raped. They gave me HIV," Ms Waithegeni said.

Now Ms Waithegeni is a physical trainer for these women, but she is also a counsellor for rape victims.

"If there is a girl, they will open up to me. I'll say to her, 'It's good to tell me', because even if you tell me a little, I will understand. I can take you to the right people for treatment. Keeping quiet only makes it harder," she said.

Some women have already made use of the new defence tactics. ( ABC News: Martin Cuddihy )

The self defence training has helped to reduce the number of reported rapes in Korogosho by 15 per cent.

Ms Nyariara has already used her skills in a home invasion, when a young man broke in at night.

"When he entered I hit him. He fell on his back. I stepped on his balls. And he said, 'Granny — don't kill me'."

Ms Nyariara said police and security have only recently arrived in the community.

Too late for too many.