The 29-year-old woman who is suspected of having run the operation is from Cameroon. She brought the women into Sweden, including a 16-year-old girl from Nigeria.

The presence and exploitation of African women in prostitution in Sweden is highly unusual. This is the largest-ever case exposed in Sweden involving women from the continent.

“She used some religious African ceremonies, curses and voodoo to gain control over the women, all of whom are poorly educated,” said chief prosecutor Krister Petersson of the chamber of the international prosecutor.

“They were then forced into prostitution, first in Malmö and then in Stockholm,” he added.

Petersson submitted the indictment to Stockholm District Court on Wednesday.

When she arrived in Sweden, the 16-year-old girl learned that she owed the 29-year-old woman in charge of the operation €60,000, or nearly 600,000 kronor ($76,150). The debt would be repaid by selling sex to Swedish men.

Even after the debt was repaid, she was forced to continue to pay half of her earnings to the 29-year-old woman, who is now charged for human trafficking. The price for the sexual services varied from 2,000 kronor ($270) to up to 10,000 kronor for an entire night.

Advertisements for the services provided by the women, who were equipped with mobile phones, were placed on the Internet. They then serviced the clients either in an apartment furnished by the 29-year-old or made house calls.

Several of the 29-year-old’s accomplices were also charged with trafficking or aiding and abetting the crime. Others were accused of pimping of varying degrees. They performed functions, such as set up web pages, driving the women to clients or renting out apartments at exorbitant prices as brothels.

The police raided the ring in December. The teenager was then cared for by social services, which arranged for a residence for her. She has since been granted permission to stay in Sweden.

About 20 clients have been identified and either been given orders of summary punishment for crimes against the law against buying sex or will be prosecuted later.

The trial begins next Monday in Stockholm District Court and will continue until November.