Two people have been released on bail as part of an investigation into slavery and domestic servitude at a house in London sparked by a report on Sky News.
The inquiry was launched after one of three alleged victims told a charity she had been held against her will for more than 30 years in a house in Lambeth, south London.
She contacted the Freedom Charity after seeing its founder Aneeta Prem in a report last summer about forced marriages.
Scotland Yard said the charity, which advises and supports victims of forced marriages or honour-based violence, got in touch and helped with sensitive negotiations, which revealed the location of the house and led to the rescue of the three women.
Police said two people detained in connection with the investigation - a 67-year-old man and a 67-year-old woman - have been bailed until a date in January, pending further inquiries.
Police believe the youngest of the alleged victims may have spent her entire life as a domestic slave.
Detective Inspector Kevin Hyland from the Metropolitan Police's human trafficking unit told a news conference at Scotland Yard that the force had "never seen anything of this magnitude".
Home Secretary Theresa May is "shocked by this appalling case," her department said in a statement.
Officers said the two suspects, who are not British, were arrested at 7.30am on Thursday and taken to a south London police station for questioning.
Detective Inspector Kevin Hyland said the victims were 'highly traumatised'
One of the three alleged victims is a 69-year-old Malaysian woman, the other a 57-year-old Irish woman and the third a 30-year-old Briton.
All three, described by police as "highly traumatised", were taken to a place of safety where they remain.
Sky's Crime Correspondent Martin Brunt said police do not believe the women were sexually assaulted, but they may have been physically and mentally abused.
Ms Prem told Sky News it was the Irish woman who phoned the Freedom Charity after watching her on television.
"I think all of them saw me on the news and made a decision because of the name of the charity and because they had seen me on TV - that gave them the courage to make that phone call," she said.
"I can't go in to too many details but they managed to get to a phone and make a call to us.
"We started to talk to them in depth when we could. It had to be pre-arranged when they were able to make calls to us and it had to be done very secretly because they felt they were in massive danger.
"It was planned that they would be able to walk out of the property. The police were on standby."
The three women were rescued from an address in Lambeth, south London
Police said the British and Irish women left the house and met police at an agreed location on October 25. They helped police find the address, where the third woman was rescued on the same day.
DI Hyland said the suspects were not immediately arrested as officers had to "establish the facts" from "extremely traumatised" victims.
He said it appeared the three alleged victims had been given "limited freedom" during the three decades they claim to have been held as slaves.
He said he was unable to confirm any relationship between the suspects and the three women who were freed.
"I don't know any relationships between the women in respect of the suspects," he said.
"Clearly, because of the nationalities of the women that have been held victims, it's very unlikely they are related in any way."
He added: "We applaud the actions of Freedom Charity and are working in partnership to support these victims who appear to have been held for over 30 years."
A neighbour said the arrested couple were "very nice".
The neighbour added: "They just kept themselves to themselves and I keep myself to myself. So it was just a case of we'd pass and say hello to each other.
"They just seemed a very normal couple. I just know it's very unfortunate."
A Home Office spokesman said: "The Home Secretary is shocked by this appalling case and while the police need to get to the bottom of exactly what happened here, she's made clear her determination to tackle the scourge of modern slavery."