Two Muslim women have accused Ramadan, a 55-year-old Oxford University professor whose grandfather founded Egypt's Muslim Brotherhood movement, of rape.
They went public with the allegations late last year when women began sharing accounts of sexual harassment and assault as part of the "Me Too" and "Balance Ton Porc" (Expose your pig) campaign triggered by the revelations against Hollywood mogul Harvey Weinstein.
Henda Ayari claims Ramadan raped her in Paris in 2012. Photo: AFP
The Geneva-born Ramadan, who took leave of absence from Oxford in November after the allegations surfaced, is the most high-profile figure to be held in France since the campaign began.
He was held for questioning in Paris for two days last week before being charged and taken into preventive custody .
The arrest of the influential academic – a regular on TV debates with more than two million Facebook followers – has rocked the French Muslim community.
Henda Ayari, a 41-year-old feminist activist who previously practised an ultra-conservative brand of Islam, claims Ramadan raped her in Paris in 2012.
Another woman, a 40-year-old disabled Muslim convert going by the alias "Christelle", claimed he raped and beat her in the southeastern city of Lyon in 2009.
Ramadan was questioned in the presence of the second woman last week.
His lawyers have suggested the women colluded to try to disgrace him.
A source close to the investigation told AFP two more women had testified anonymously against him and could file formal complaints.