Egyptian police have arrested Gehad El-Haddad, the Muslim Brotherhood spokesman, according to security officials.
Haddad was detained with two other Brotherhood members in an apartment in the capital Cairo on Tuesday, the officials said.
He served as chief of staff to Khairat El-Shater, the Muslim Brotherhood’s deputy leader, and is the son of Essam El-Haddad, an aide to deposed President Mohamed Morsi.
Haddad, the Brotherhood’s main point of contact with international media before the crackdown, is accused of inciting the killing of protesters.
The two others detained along with Haddad are Hossam Abu Bakr, a provincial governor under Morsi, and Mahmoud Abu Zeid, a member of the Muslim Brotherhood’s executive board.
Also on Tuesday, an Egyptian court upheld an order from the prosecutor’s office freezing the assets of 25 leading members of the Brotherhood and allied parties.
Many of the group’s senior leaders have been detained since the army deposed Morsi on July 3, on charges similar to the accusations levelled against Haddad.
In a separate development, France’s foreign ministry said a Frenchman died on Friday while in police custody in Cairo after being arrested for breaking curfew.
It is believed he was beaten to death in his cell by other detainees in a dispute over whether to turn the lights on.
The victim had been residing in Egypt and the family in France had been contacted, he said.
At least a thousand people have been killed in Egypt since the July 3 toppling of Morsi, comprising mostly his supporters but also scores of members of the security forces.
Morsi, who is being held at an undisclosed location, has himself been charged with inciting killing and violence.
Besides Morsi, the Brotherhood’s three top leaders are also in jail, together with the head of its political party.
It is the toughest crackdown on the Islamist group in decades.