The 51-year-old was held in custody over allegations relating to atrocities during the Liberian civil war between 1989 and 1993.
The woman was arrested just after 7am on Thursday,
Police said that searches are being carried out at two addresses in east and central London.
A Met Police spokesman added: "We continue to liaise with the Foreign and Commonwealth Office and Crown Prosecution Service regarding this investigation."
The Liberian civil war raged from 1989 when government minister Charles Taylor started an uprising in a bid to topple the government.
Backed by a rebel group, the National Patriotic Front of Liberia (NPFL), he gained control of large areas of the country and rose to become one of Africa's most prominent warlords.
The NPFL has been accused of a wide range of human rights abuses and the large numbers of deaths during the conflict eventually led to the involvement of the UN and the Economic Community of West African States.
A final peace agreement led to the election of Taylor as President of Liberia in 1997.
A second civil war broke out in 1999 and Taylor was forced into exile in 2003.
He was later jailed for committing war crimes in neighbouring Sierra Leone.
Additional reporting by Press Association
