Petitioning Members of Parliament and appearing on morning shows she asked people to speak out on an issue that affects one in three around the world.
"We are talking about everyone's lives are touched by this issue, whether it be our children, or our mothers or our sisters. So many women. The most important thing I can do here today is start this dialogue that we're having," Witherspoon, 33, told the show GMTV.
Later, Witherspoon went to the historic Houses of Parliament, where she made a brief address and listened to testimonies of abuse.
"She looked quite emotional, she kept looking to the floor whilst she was listening to the other speakers," an onlooker said "She seemed almost tearful."
Witherspoon said she felt "more than a little overwhelmed to be here today."