Hazel Thompson has spent 11 years investigating the red light district of India's biggest city. In a new book she gives a voice to girls who were taken from their homes, raped, caged and sold for sex

It was pitch black as I stumbled through the labyrinth of the dark corridors of a large brothel house in Kamathipura, Mumbai's notorious red-light district. I'd been told to hide my camera under my scarf, not to speak and not to make eye contact with anyone. With my hand I felt the filthy walls dripping with condensation from the intense heat.

Eventually, guided by my Indian fixer, I came to a dimly lit door at the end of a corridor. Like a prison guard, an ageing madam came to the front of the brothel and unlocked the large padlock with her set of keys. I was taken into the reception area of the brothel, the space where the customers are taken to select a girl. In the ceiling I could see a small, open trap door. When the madam had disappeared, I climbed up a wooden ladder and pushed through the small gap.

Suddenly I was face to face with a "box cage". I knew what I was looking at. The prostitutes had told me of the caged rooms and boxes they had been held in for months, even years, when they were first taken and trafficked to the red-light district. The madams would keep the girls like slaves in the cages until they were "broken", to the extent that they would not try to run away. The girls told me they never knew if it was night or day. They were only taken out to eat or to be given to a customer for sex. For years I had wanted to photograph these cages, to prove that these places actually exist.

I was returning to Kamathipura 11 years after my first visit – ostensibly as an aid worker. In reality I was using the access I got to make a photographic and video record of a world that shouldn't be allowed to exist. I did a lot of hiding in the back of brothels, trying to chronicle the lives of girls trafficked and tricked into sex slavery. Before 2002, when I first came to India to photograph children born into this vile industry, I knew nothing about trafficking. Those early days of visiting Kamathipura changed my life, and I've been unable to let this story go.

I was unable to let go because of girls such as Guddi. I first met Guddi while I was shooting on 14th Lane, in the heart of Kamathipura. I was sheltering from the intense summer heat in the charities clinic when Guddi came rushing through the door. She was distraught and crying after being beaten by one of the gangsters who run the district.

I didn't understand what she was saying as she explained the problem to one of the charity's staff, but I could see her pain and brokenness as tears trickled down her childlike features. I witnessed the shocking sight of her badly bruised legs as she pulled up her skirt to show us the results of her beating. And I remember watching her as she told us what had happened. I remember thinking how incredibly beautiful this girl was, and how she looked so out of place. She had a poise and an innocent beauty that belonged more on a Bollywood poster than in the dark lanes of Kamathipura.

Her youth and vulnerability showed me how the hardened women also lining the streets of the red-light district were also once little girls whose childhoods had been stolen from them. From that day in the clinic, Guddi and I slowly became friends. She stood looking for customers on the same corner each day, and every time I entered the red-light district she would greet me. Over time she started to open up and tell me her story. That tale, and others, different but just as brutal, became the basis of the book.

Guddi was just 11 years old when she was trafficked. She was lured directly from the protection of her parents and 13 other siblings, from her home in a poor village near Kolkata. Her trafficker was her mother's friend, who had lived next door to her family home all her life. She promised Guddi well-paid domestic work in Mumbai that would help feed her struggling family. Guddi's life dramatically changed as soon as she arrived in Mumbai. She was taken to Kamathipura, where she was dragged into a brothel on 14th Lane and raped by a paedophile customer, while the madam and her daughter held her down by her arms and legs to restrain her. The customer raped her so violently that she was hospitalised for three months. They raped her to break her. She was then held, not knowing if it was day or night, in a caged room in a brothel house on 14th Lane, the same street she still lives in.

Every time I return to Kamathipura, I never know if I will find Guddi still standing on her corner. I always hope she has escaped, and then I also dread that if she is not in her usual place, the reason may be that she hasn't survived a beating. The last time I said goodbye to Guddi, she wept in my arms. She asked me not to forget her. But how could I forget this precious life, whose heart I could feel beating against my chest as she cried? I begged her to leave as I feared for her life. I didn't want her to die there and believed that she did have a future out of this place.

All she could say to me was: "I am trapped on all sides, sister: my life was taken when they brought me here."

Her phrase gave the ebook its title – Taken. Guddi's face is on the cover. She stands for so many women's lives and for their families' lives. Lives that have been destroyed by sex trafficking and slavery. My ambition is that Taken, showing the reality of a red-light district in all its dangerous and sordid dimensions, will inspire people to help put an end to this modern-day slavery. I was given incredible trust and access to tell these women's stories. I just hope I have provided a platform that can give these voiceless victims a voice.

Sex slavery in India has deep links with Britain's history: the system was originally set up by the British military. Take a look at The Queen's Daughters in India. It is a report of two women who, in 1892, went undercover as aid workers, just like I have, investigating and interviewing the girls trapped in sex slavery more than 120 years ago. When I read this book I wept, because I was reading the same stories – of girls being sold, tricked and trafficked into prostitution.

Taken will be published as an interactive ebook, but I am also working with Danny Smith from the Jubilee Charity, which campaigns for children at risk. We are launching together the Taken campaign, which includes a petition calling on India and other countries to criminalise the purchase of sex and stop criminalising women. Legislation passed in Sweden in 1999 made the purchase of sex illegal, and that has dramatically reduced the number of women trafficked into sex slavery. It also stops victims of trafficking being criminalised by the police and the legal system. The sex trafficking of women and girls is widespread in India, and on the rise. We are inviting people to join the Taken campaign in calling for change by signing our petition.

Over the past 11 years, I have watched and photographed thousands of men visiting Kamathipura to find pleasure. But a red-light district is not a place of pleasure. It is a place of pain. I have often wondered if men would come and have sex with these women if they knew the true stories of how they have been trafficked, and that actually they are paying to rape a girl who is a sex slave. Taken tries to tell some of those stories.

Taken: Exposing Sex Trafficking and Slavery in India is available for £7.99 from iBooks on iPad, iPhone or iPod Touch or at iTunes.com/iBookstore, from 9 October. For more information about the ebook and campaign visit takenebook.com. Hazel Thompson will donate the profits from the sales of each Taken digital book to the British charity Jubilee Campaign, which has set up the Taken campaign to rescue and rehabilitate girls and women from Mumbai's sex industry