In her speech to the Labour Party Conference in Brighton in her brief as Equality Minister, Harman said the Government was "stepping up" preventative measures against human trafficking.
She said: "We're determined to ensure that, especially in the run up to the Olympics, international criminal gangs don't trick and abduct women from abroad and sell them for sex in London."
Police in London have already started working to prevent an increase in human trafficking in the build-up to the 2012 Olympics and Paralympics, a new report published in July revealed.
A report by the Metropolitan Police Authority (MPA) published in July warned that the Games could bring an increase in prostitution and sex trafficking as well as domestic violence and rape, caused by alcohol abuse.
These crimes could damage London's reputation in the eyes of the world, as attention focuses on the nation's capital during the Games, the report said.
The hard-hitting warnings were presented in a report presented to the police authority by Catherine Crawford, the chief executive of the MPA.
She recommended that the Met be asked to set out its plans for tackling the potential increase in crimes against women.
But she also reported that officers had already begun work to try to prevent people trafficking in the run up to 2012.
Police in the five Olympic Host Boroughs of Greenwich, Hackney, Newham, Tower Hamlets and Waltham Forest had already noticed "a slight increase" in the number of women being trafficked to Britain to work in the sex industry, she said.
But research conducted by the European Union (EU) had warned that major sporting events created a ''temporary and spectacular increase in the demand for sexual services".
During the Athens Olympics in 2004, the number of human trafficking victims almost doubled.
London MEP Mary Honeyball had launched a call more than a year ago for a Europe-wide action plan after an estimated 3,000 women were sold into the sex trade and smuggled into Switzerland ahead of the Euro 2008 football championships.
August 2009: Mobile phone companies asked to help on 2012 crackdown
July 2009: Police already working to clean up London before 2012 Games