On the porch of a safe house in Liberia's capital city, Monrovia, six year-old Mercy is playing a card game. As her tiny hands shuffle the frayed cardboard squares, Mercy explains how she came to live within the safe house's high-security fences. Two weeks ago, she says quietly, she was walking along the road on her way to visit her father at work when a "big man" grabbed her. The stranger took her to a derelict house where he kept her overnight and raped her, repeatedly. She was found the next morning and taken to hospital.

Mercy is now on a course of powerful drugs to stop her contracting HIV and other sexually transmitted infections she may have been exposed to during the attack. She will stay in the charity-run safe house, guarded by female security staff, until it is considered safe for her to return to her family. "Most of the time, the community blames them for what happened to them," Mercy's social worker, Marriam, says. "The family may even disown the child for speaking out."

Thousands of young girls like Mercy are raped every year in Liberia. A quarter of the victims are under five years old. There are no official figures estimating the extent of the problem: in a war-torn country with no mains electricity, no running water and 85% unemployment, the authorities have more immediate concerns than gathering accurate statistics. But Liberia's free clinics are overwhelmed with child victims, and doctors from Medecins Sans Frontières have speculated that up to 10,000 Liberian children must be victims of rape every year – in a country with a population of little over 3 million.

It is easy to view the crisis as a consequence of Liberia's brutal 14-year civil war, in which both Charles Taylor's government forces and rebel factions used rape as a weapon to terrorise and destroy communities. Yet Liberia has been at peace since 2003 – only a few months after Mercy was born – and Liberians are at a loss to explain why this particular legacy of war appears to be getting worse, and children of the new, peaceful, democratic Liberia are now being victimised on such a shocking scale. "When you follow the rape cases in Liberia, it's like the men are going mad," says Marriam. "They are just possessed. The way they are doing the rape business, they are confusing us."

Children are viewed as potential sexual conquests all over the country – and not just by Liberian men. Lofa county saw some of the war's most intense and prolonged fighting, and children orphaned by the conflict must now beg for their survival here, but they often find those they approach for food, money or help expect sex in return. In Foya, near the borders with Guinea and Sierra Leone, Oretha, 15, and her sister Sarah, 16, have been having sex – or "man business", as they call it in Liberian English – since the ceasefire, when Oretha was nine. When they are hungry, they say, they go to the town's main highway and beg from foreign aid workers in NGO-branded 4x4s who give them the equivalent of 40 pence in exchange for sex. If there is no one on the highway, they go to the base where the UN peacekeepers are stationed and ask for food, but they say the peacekeepers, too, expect sex in return. The UN says it takes allegations like these very seriously and has promised to investigate them. It acknowledges there have been problems of sexual abuse and exploitation by UN peacekeepers in Liberia, including some that allegedly involve underage girls.

Liberia's president and Africa's first female head of state, Ellen Johnson-Sirleaf, was elected in 2005, vowing to tackle sexual violence. She has installed a female chief of police, a team of women prosecutors and a female judge who presides over a criminal court specifically dedicated to trying rape cases. But four years on from Johnson-Sirleaf's historic win, justice for rape victims seems agonisingly slow. It can take years for cases to come to trial. The dedicated court started hearing cases in May this year, but took two months to try its first case.

In Monrovia Central Prison, the largest correctional facility in the country, there is only one prisoner who has been convicted of child rape. When asked about the details of his crime, 58-year-old Alfred Brooks shuffles uncomfortably on the bench in the prison superintendent's office, protesting his innocence. But for a man who denies having sex with underage girls, Brooks had plenty to say about why men are drawn to them. "The girls here make you have sex with them," he says, simply. "They entice you, they induce you." If young girls stopped going around "partly naked", he adds, men might view them differently.

The minister of gender, Vabah Gayflor, concedes that the response to rape cases in Libera has been disappointing. "We hope that something dramatic could happen where changes could come about so these young girls could have an opportunity to grow up in an environment where they can be safe," she says. But she stops short of suggesting what any of those changes might be.

As long as it is commonplace for Liberian children to be viewed as sexual objects, and for men to think they can take what they want from them with impunity, then the child rape problem will persist and grow. Until the problem is properly addressed, the country cannot move forward: Liberia has little hope of escaping the horrors of the past while they are being revisited upon the very youngest members of Liberia's next generation.

Unreported World, Liberia: Stolen Childhood, will be broadcast on Friday 16 October at 7.35pm on Channel 4