The Oxford professor was taken into custody on Wednesday as part of a preliminary inquiry in Paris into rape and assault allegations, with a legal source saying investigators wanted to question him further on Thursday morning.
Ramadan, a Swiss national whose grandfather founded Egypt's Muslim Brotherhood Islamist movement, has furiously denied separate accusations from two Muslim women that he raped them in French hotel rooms in 2009 and 2012.
A regular face on French television, the 55-year-old academic is the most prominent figure to be held in France over the sexual assault and harassment claims that have rippled around the world as a result of the online "Me Too" campaign.
But in October, she said she had decided to name Ramadan publicly as the alleged perpetrator as a result of the "Me Too" campaign -- which in France is known as "Balance Ton Porc" (Expose your pig).
She said Ramadan raped her in his hotel room, telling Le Parisien newspaper: "He choked me so hard that I thought I was going to die."
She lodged a rape complaint against Ramadan on October 20.
In November, Oxford University said Ramadan was taking a leave of absence from his post as professor of contemporary Islamic studies, "by mutual agreement".
He has denied the two women's accusations, as well as further allegations in Swiss media of sexual misconduct against teenage girls in the 1980s and 1990s, denouncing them as "a campaign of lies launched by my adversaries".
Lawyers for the married father-of-four have accused Ayari of slander and suggested the women colluded to try and disgrace him.
Both women have already been questioned by police in Paris and the northern city of Rouen.
The accusations have sparked heated online debate between supporters of the telegenic Ramadan, who commands a following of more than two million fans on Facebook, and his opponents.
Ayari was placed under police protection in November after receiving death threats.
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