Muslim Brotherhood General Guide Mohamed Badie was arrested early Tuesday in Cairo, becoming one of the most senior members of the Islamist movement yet to be detained by the Egyptian security forces.
Badie, the Brotherhood's spiritual leader, was taken into custody at an apartment in Nasr City, northeast Cairo, state media reported.
He is accused of complicity in the deaths of eight protesters outside the Brotherhood's Cairo headquarters in June, according to Al Jazeera.
With Badie's arrest, almost all of the Muslim Brotherhood's senior leadership is now in custody, Al Jazeera says. Badie's deputy Khairat al-Shater, another influential figure, was arrested shortly after the military takeover on July 3, along with scores of other Islamists.
The Interior Ministry published photos of Badie in detention on its Facebook page, while satellite TV broadcast live footage of the 70-year-old being guarded by armed police.
The pictures are the military-appointed interim government's attempt to humiliate the Brotherhood and ensure its negotiating position is weak, according to BBC analysts.
Badie is said to have gone into hiding soon after the military deposed President Mohamed Morsi and ordered the arrest of dozens more Brotherhood figures in early July.
His son, Ammar Badie, was killed by gunfire during a Brotherhood protest in Cairo's Ramses Square on Friday. The movement had called for a nationwide "Day of Rage" in response to the security forces' crackdown on its protest camps two days earlier, when more than 700 people died.
In total almost 900 people are thought to have died in Egypt since the crackdown on August 14, most of them civilians but also some 100 police and soldiers.
Judicial authorities on Monday extended Morsi's detention by another 15 days. Meanwhile his unelected predecessor, Hosni Mubarak, has been cleared of almost all corruption charges against him and could be released as soon as this week, his lawyer said.