Mariatu was 15 years old when her widowed mother died and she ended up sleeping rough on the streets of Freetown where she fell into commercial sex work.

Not long after, she was arrested for “loitering” and, unable to pay a police bribe, spent six months in an adult jail.

Now 22, she is desperate to escape a life marked by suffering, violence and exploitation and is appealing to the state to provide more support to Sierra Leone’s sex workers who are stigmatised by society and suffer unfair treatment at the hands of the legal system.

She spoke to the Guardian before the online launch of a documentary to mark the International Day to End Violence Against Sex Workers, which highlights the harassment and violence faced by the marginalised group.

The film features interviews with commercial sex workers – including girls as young as 12 – and examines their treatment by Sierra Leone police officers who are accused of exploiting them for sex and money.

Between five and 15 sex workers are arrested every month in Freetown, and many are forced to pay bribes to avoid jail, according to AdvocAid, which provides free legal representation and support to vulnerable women.

“The documentary shows how archaic laws such as ‘loitering’ and ‘frequenting’ are often used to target sex workers,” says its executive director Rebecca Wood.

Its not just their liberty that is at risk. Sinneh Kamara, a coroner forensic technician working in Freetown, says every month he buries more than 10 sex workers aged between 13 and 26. “They die from HIV, they die from cold, bronchitis and pneumonia.”

Mariatu was first arrested aged 15 for “loitering”, which is vaguely defined under the Public Order Act 1965 as an offence whereby someone is “in the open air” without a visible purpose.

“The police demanded I pay 300,000 leone [£30] – the equivalent of a month’s wages,” she recalls. “Because I did not have the money they charged me and I was imprisoned for six months.”

She now hopes to train as a seamstress but would take any job over working on the streets. The second youngest of 11 daughters, Mariatu left school at eight because her mother couldn’t afford the fees after her father’s death during the war.

Many of Sierra Leone’s estimated 26,000 sex workers come from poor backgrounds and entered the trade as minors. A lack of education and training makes it hard to break out of the cycle and find other employment.

Mariatu makes around 10,000 to 15,000 leone [$1-$1.50] per client and sees up to 10 clients a night. “Either men invite me into their homes or I find a dark spot away from the traffic and lay down some material that I carry with me in the street,” she says. “I’ve seen a lot of violence and been beaten on several occasions. Frequently I’ve had all my money stolen.”

She was jailed for second time at 17 in a case of mistaken identity, after another sex worker was accused of stealing money from a client. She spent two weeks in custody before being appearing in court and having her case discharged with the help of AdvocAid.

On countless occasions she has been picked up by police and asked for sex in return for her release. “Each time I give them money instead or beg them to let me go. I would rather spend the night in the cell than have sex with them.”

Police exploitation is commonplace according to testimonies made in the documentary entitled “Kolonko”, a Krio word for sex worker.

One woman says: “The worst thing is when you earn your money working all night and the police arrest you and you have to pay bail. What you will eat in the morning? They leave you hungry.”

Another sex worker explains: “The police abuse us for our money and if you don’t have the money they want to have sex with you. And if you don’t have sex with them, they will put you in a cell.”

Emmanuel Sattie, national human rights coordinator at Amnesty International, tells filmmakers: “Most of the crimes levied on these girls relate to loitering. One of our concerns is if the [the police] are so keen on targeting sex workers why do they not also go after the men that are part of the trade.”

One woman describes how she has been thrown in prison more than 10 times for “loitering” and even given birth in a cell. “They said someone killed a policeman and because I was in the area they arrested me. I was eight months pregnant and my son was born in jail.”

Sex work is illegal in Sierra Leone, making it difficult to challenge the way the legal system treats those who rely on it for a living. The lack of protection from police means sex workers distrust the law and find it difficult to report crimes against them.

Julie Sesay, programme manager at Advocaid, says: “Despite prostitution being illegal, the inspector-general of Sierra Leone police has told us that in all his time with the force he has never heard of a women nor a client who has been prosecuted for sex work.

“It’s always other types of offences that women are arrested for. And we know of no instances where men – either involved in the trade or those who pay for sex – have been arrested.”

She said the treatment of sex workers by police briefly improved after Kolonko was screened in Freetown last year.

AdvocAid is hoping that the film’s international platform will help it to push for legal reforms and pressurise the police into changing their behaviour.

Wood says: “We are in discussions with police leads about what steps they are taking to professionalise the force. We are also trying to push for the Public Order Act to be revisited so offences like loitering, that leave the police with very broad powers to make arrests, are decriminalised.”

In the meantime, Mariatu is still trapped in the trade and vulnerable to all risks including violence and sexually transmitted disease.

She says: “When I go out at night I feel very vulnerable. I’ve seen many friends die from illness and I know I will die too if I don’t get off the streets.”

The Guardian contacted the Sierra Leone Police for a response but they declined to comment.