Ramadan, whose brother is the intellectual Tariq Ramadan and whose grandfather founded Egypt's radical Muslim Brotherhood, was arrested in Colmar, eastern France, while attending a conference.
He was "known in the past to have adopted behaviour and made remarks which pose a serious threat on French soil," the ministry said in a statement yesterday.
"The interior ministry and the forces of law and order are fully mobilised and will continue to fight ceaselessly against extremism and radicalisation," Interior Minister Matthias Fekl said in the statement.
Six years later he won 345,000 Swiss francs compensation over the sacking.
Ramadan's brother Tariq is banned from entering the United States.
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