The victims, who were lured to the Spanish mainland with fake promises of jobs, were forced to work 14 hours a day. One of the women, police have revealed, was just 16 years of age.
All 21 are Nigerian and were working in the exclusive districts of Ibiza.
A major operation carried out by the Spanish police in association with the Office of Criminal Investigation in Germany and Europol has led to the arrest of 24 suspects.
'The network captured very young victims among the lower classes of the major Nigerian cities, deceiving them with false job offers in Spain.
'Once in our country, they were forced into prostitution in marathon days, being beaten if they did not earn the money demanded by the gang,' said a police spokesman.
Investigators said Ibiza was chosen for the summer because of the high influx of tourists, with the gang totalling controlling the streets of the exclusive areas.
They were kept in one apartment and only allowed out occasionally to buy food or to keep an appointment with a client.
'If they didn't earn 1,000 euros a day, they would be forced to kneel for hours and beaten with sticks and brooms,' said the spokesman. The vulnerable women were given employment offers 'too good to refuse' in view of their poor circumstances but once captured, were unable to escape.
They were subjected to voodoo rituals and sworn to loyalty contracts under the threat of family members being killed. The women were smuggled into Europe on boats and planes and were only told their job offer was false when they arrived in Spain.
They were then told they would have to pay up to 50,000 pounds to be freed and could do so through prostitution.
Those arrested included two women said to be the ringleaders who had 'years of experience' in recruiting vulnerable girls.