Egypt's military-backed authorities on Tuesday arrested the supreme leader of the country's Muslim Brotherhood, dealing a serious blow to the Islamist group at a time when it is struggling to keep up its street protests against the ouster of President Mohammed Morsi in the face of a harsh government crackdown.

The Brotherhood's spiritual guide, Mohammed Badie, was arrested in an apartment at the eastern Cairo district of Nasr City, close to the location of the six-week sit-in protest by supporters of Morsi, who also hails from the Islamist group. The encampment was cleared by security forces last Wednesday, along with another protest site in Giza, in a raid that killed hundreds of people.

Badie's arrest is the latest stage in an escalating crackdown by authorities on the Brotherhood in which hundreds have also been arrested. The Brotherhood's near daily protests since Morsi's ouster have somewhat petered out the last two days, with scattered demonstrations in Cairo and elsewhere in the country attracting hundreds, sometimes just dozens.

Morsi himself has been detained in an undisclosed location since the July 3 coup, prompted by days-long protests by millions of Egyptians demonstrating against the president and his rule. He is facing accusations of conspiring with the militant Palestinian Hamas group to escape from prison during the 2011 uprising and complicity in the killing and torture of protesters outside his Cairo palace in December.

Badie's last public appearance was at the sit-in protest last month, when he delivered a fiery speech from a makeshift stage in which he denounced the July 3 military coup that removed Morsi.

Badie's arrest followed the death of one of his children, son Ammar, who was shot dead during violent clashes between security forces and Morsi supporters in Cairo on Friday.

Also, Badie and his powerful deputy, Khairat el-Shater, are to stand trial later this month on charges of complicity in the killing in June of eight protesters outside the Brotherhood's national headquarters in Cairo.

Security officials said Badie was taken to Torah prison in a suburb just south of Cairo and that a team of prosecutors were to question him Tuesday. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

Torah is a sprawling complex where the autocrat Hosni Mubarak, ousted in the 2011 popular uprising, is also held, along with his two sons. Several Mubarak-era figures are also imprisoned there, as are several Brotherhood leaders and other Islamists.

In the aftermath of last Wednesday's violence, the military-backed government is considering outlawing the Brotherhood, which has spent most of the 85 years since its creation as an illegal organization. The government has asked the judiciary for advice on how to go about a ban. It has also come under growing pressure from the pro-government media and a wide array of secular politicians to declare the Brotherhood a terrorist organization.

Brotherhood spokesman Ahmed Aref sought to downplay the significance of Badie's arrest, writing on his Facebook page on Tuesday simply: "Mohammed Badie is one member of the Brotherhood."

The private ONTV network showed footage of a man the network said was Badie after his arrest. In the footage, a somber looking Badie in an off-white Arab robe, or galabeya, sits motionless on a black sofa as a man in civilian clothes and carrying an assault rifle stands nearby.