Some 700,000 women are trafficked to western Europe every year, said lawyer Birgit Thoma, who works for Solwodi, or Solidarity with Women in Distress.
Affordable transport and instant communication have led to an increase in trafficking over past 10 years, with the trade now worth an estimated $30 billion (18.8 billion euros) globally, according to a United Nations report.
For many years the focus was on human trafficking from eastern Europe, but when the EU expanded -- mainly to the east and south -- in 2004, the legal status of women in the new member states changed. That's led Solwodi to shift its focuses to African women who are forced into prostitution in Europe.
Thoma said foreigners make up some 70 percent of people in Germany's sex trade. While exact figures aren't available, she estimated that about 100,000 women from Nigeria alone have been trafficked to western Europe.
Women victimize women
Unlike tactics used in eastern Europe, African women are often lured with marriage deals. The traffickers don't belong to large mafia gangs, but are organized in smaller, inconspicuous networks.
"Often the criminals are women," said Thoma. "These are the so-called 'mesdames,' most of whom used to be victims themselves."
Voodoo rituals are often used to scare and psychologically intimidate the women, she added.
"Priests force them not to say where they're going and what happens to them," Thoma said. "Otherwise something will happen not only to their families, but sickness, death or curses will come over them too."
In 1985, the Catholic nun Lea Ackermann founded Solwodi in Kenya to assist women whose financial desperation had led to a life of prostitution. Three years later, the first Solwodi branch was founded in Germany as a refuge for foreign women who had become victims of forced prostitution or trafficking.
Europe revises legal framework
Prostitution is legal in Germany, which creates obstacles to uncovering and prosecuting cases of trafficking. Since around 30 percent of trafficked women were aware beforehand that they would end up working in the sex trade, it is difficult to collect evidence proving they were forced into prostitution, Thoma said.
However, forced prostitution was redefined in 2005 when EU standards were applied to German law. As a result, human trafficking for the purpose of sexual exploitation is no longer a sex crime but a "crime against physical integrity and against freedom," Thoma explained.
She added that the law's inclusion of robbing people of their freedom was a better description of forced prostitution than labeling it a sex crime.
Germany is not alone in rethinking its laws surrounding prostitution. Sweden was the first in Europe to outlaw paying for sex in 1999. Last week, Norway's government proposed to fine or jail clients of prostitutes for up to six months in an effort to counteract trafficking and lower demand.
In Britain, where paid sex is legal but prostitutes aren't allowed to solicit in public, a group of Labour MPs have advocated for replacing criminal penalties for street prostitutes with mandatory counseling programs to get them out of the business.
"We don't criminalize people who sell kidneys, we criminalize the buyer," Labour MP Fiona MacTaggart told Reuters news agency.
Address the problem at its roots
The justice system also makes it difficult prosecute traffickers who force women into marriage. Victims of this crime have to prove that they suffered threats or abuse -- not only that they forced to marry against their will. These women also risk penalties if they are shown to have married only to acquire a residence permit.
For those without residence permits, a new law in Germany aims to encourage them to testify against their traffickers. After the initial three-month tourist visa, trafficking victims are granted an additional six months to consider whether to press charges.
"If they don't testify, they're deported," said Thoma. "But if they testify, they get a residence permit for the duration of the criminal proceedings."
But ultimately, trafficking needs to be addressed from the bottom up, said the lawyer. That means pulling the women out of poverty and offering them a chance to improve their lives.
"We have to create more possibilities for education there and improve the overall living situation for the women," she said.