Egyptian security forces arrested in the small hours of Wednesday senior Muslim Brotherhood leader Essam al-Arian.
The Interior Ministry said al-Arian was arrested in an apartment owned by one of his friends in al-Tagamoaa al-Khamis in eastern Cairo.
The senior Islamist showed no sign of resisting his arrest, said the ministry.
Assistant Interior Minister for Security Gen. Ahmed Helmi described the arrest of al-Arian as a "serious blow" to the Muslim Brotherhood.
The arrest came a few days before the trial of ousted president Mohamed Morsi, scheduled to start on Monday.
The Interior Ministry said al-Arian was taken to the Tora prison complex in southern Cairo.
He will be referred to the prosecution on charges of inciting violence against protestors.
Sources with the Muslim Brotherhood confirmed the arrest, but tried to play down its importance.
"Al-Arian had no influential field or partisan role," the sources told Anadolu Agency.
"His arrest will not affect the National Alliance for the Defense of Legitimacy, the Muslim Brotherhood or its Freedom and Justice Party (FJP)," they claimed.
Al-Arian was the vice-president of FJP.
Most top and medium-level leaders of the Muslim Brotherhood and the FJP have been arrested since the powerful military ousted Morsi, Egypt's first democratically elected president, on July 3.
They face charges of inciting violence and murder, which they dismiss as politically-motivated.
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