Police in Spain say they have arrested 21 people and freed four women while breaking up an alleged human trafficking ring.

The country's national police said in a statement Saturday that the bust in the southern city of Malaga happened after a victim called an anti-sex trafficking hotline saying she was being forced into prostitution and to consume illegal drugs.

Police say the ring was led by a woman who operated four locations where victims were forced to prostitute themselves in "marathonic sessions."

One of the freed women first was exploited when she was a minor.

Spanish police say the anonymous hotline established three years ago has received more than 1,800 calls this year, leading to about 400 interventions by the police.