Egypt’s military-backed authorities on Tuesday arrested the supreme leader of the 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 Morsy 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 by supporters of Mr. Morsy, 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.

Mr. Badie’s arrest is the latest stage in an escalating crackdown by authorities on the Brotherhood . The Brotherhood’s near daily protests since Mr. Morsy’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.

Mr. Morsy 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. Mr. 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.

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

Also, Mr. 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.

Mr. Badie’s arrest came after suspected Islamic militants ambushed two minibuses carrying off-duty policemen in Egypt’s Sinai Peninsula early on Monday, forcing the men to lie on the sand and shooting 25 of them dead.

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