The Egyptian capital Cairo has been described as the "most dangerous" megacity for women.
The finding comes from the first international poll on how women fare in cities with over 10 million people.
The survey was conducted in 19 megacities where experts on women's issues were asked about how well women were protected from sexual violence.
The poll ranked London as the megacity most friendly to women, followed by Tokyo and Paris.
London Mayor Sadiq Khan said women were now leading in every level of society in London, including public service, the arts, business and politics.
Women's rights campaigners in Cairo blame centuries-old traditions for the discrimination there, with any progressive steps to help women extremely difficult.
Women also have limited access to good healthcare, finance and education.
Shahira Amin, a high profile Egyptian journalist, said everything about the city was difficult for women, and even something as simple as walking down the street could expose a woman to harassment and abuse of all kinds.
Karachi, Pakistan, Kinshasa in the Democratic Republic of Congo and the Indian capital Delhi come behind Cairo in the survey, conducted by Thomson Reuters Foundation.
Delhi and the Brazilian city of Sao Paulo were reported to be the worst for risk of sexual harassment, sexual violence and rape.
The figures for Delhi come despite tougher laws on sex crimes after a fatal gang rape of a woman on a bus in Delhi in 2012 which led to popular protests.
A poll conducted for the Brazilian Forum of Public Security found one in three Brazilian woman 16 or over had suffered some of form of physical, verbal or psychological violence.