One of the women believed to have been held captive in a house in south London had no contact with the outside world for her entire life, police believe.
Two people - a man and a woman both aged 67 - have been arrested accused of keeping three women against their will in a property in Lambeth for more than 30 years.
The women, a 69-year-old Malaysian, a 57-year-old from Ireland and a 30-year-old British woman, were rescued from the house prior to the arrests.
Police said the victims were deeply traumatised and believed that the youngest had lived in the property for her entire life with no contact with the outside world.
Police believed the woman had "controlled freedom" inside the house but nothing more.
"We've never seen something on this magnitude before," Det Insp Kevin Hyland said.
Police arrested the two suspects - who are not British - after they were contacted by Freedom Charity where staff had received a call from a woman who said she had been held captive for the past 30 years.
Aneeta Prem, founder of charity, said the three women were "absolutely terrified" by their alleged captors.
She told Sky News: "They felt they were in massive danger.
"I don't believe the neighbours knew anything about it at all. It was just an ordinary house in an ordinary street.
"They were very restricted on everything they could do.
"I just can't believe that human beings could treat each other that way. It's something that's so barbaric and so out of anyone's imagination that anybody could treat anybody in such a way that people feel that they are slaves, if you like.
Hyland said: "We applaud the actions of Freedom Charity and are working in partnership to support these victims. We have launched an extensive investigation to establish the facts surrounding these very serious allegations.
"A television documentary on forced marriages relating to the work of the Freedom Charity was the catalyst that prompted one of the victims to call for help and led to their rescue."