Egyptian security forces have arrested Mohamed Badie, the Muslim Brotherhood's spiritual leader, at a residential apartment in Cairo.
He's accused of inciting violence.
Over 900 people have been killed in a week of violence in Egypt - sparked by clashes between security forces and the Muslim Brotherhood.
Meanwhile, Irish Diplomats still haven't managed to locate the four Irish siblings who were detained after security forces stormed a mosque in Cairo at the weekend.
The envoys visited Tora prison and several other facilities on the outskirts of the city, in an unsuccessful attempt to locate the three sisters who are in their twenties, and their teenage brother.
The four are children of Hussein Halawa, the Imam at Ireland's largest mosque in Dublin.
In the latest bloodshed, militants killed 25 policemen in the Sinai Peninsula, just hours after 37 Muslim Brotherhood prisoners died in police custody.
The interior ministry said police picked up Brotherhood chief Mohamed Badie, 70, near Rabaa al-Adawiya square, where more than 200 Morsi supporters were killed on Wednesday as police cleared their protest camp.
Senior members of the Muslim Brotherhood organisation, including Badie, have been accused by Egypt's military-backed authorities of inciting the violence that the deaths of protesters.
And judicial sources said fresh accusations had been levelled against Morsi himself, who has been held in a secret location since the military deposed him on July 3.
Diana Eltahawy of Amnesty International says they are not confident of Egypt's ability to investigate what happened: