CAIRO – Egypt’s military-backed authorities arrested the Muslim Brotherhood’s supreme leader on Tuesday, dealing a serious blow to the embattled movement at a time when it is struggling to keep up 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 in the Cairo district of Nasr City, close to the site of a sit-in encampment that was forcibly cleared by security forces last week, triggering violence that killed hundreds of people.

Badie’s arrest is the latest move in an escalating crackdown by authorities on the Brotherhood.

The Muslim Brotherhood said Badie’s detention would not weaken the movement or lead its followers away from their principles. “The people will continue their peaceful struggle until they regain all their rights with his eminence, the guide (leader) in jail,” it said.

The group’s near-daily protests since Morsi’s ouster have diminished in recent days, with scattered demonstrations in Cairo and elsewhere attracting mere hundreds, or even dozens, of protesters. On Tuesday, several hundred Morsi supporters staged protests in Helwan, an industrial suburb north of Cairo, and in Ein Shams, a residential district on the opposite end of the city, shortly before the 11-hour curfew went into effect at 7 p.m.

Morsi has been detained in an undisclosed location since the July 3 coup that ousted him, following protests by millions of Egyptians against his rule.

Badie’s last public appearance was at the Nasr City protest encampment last month, where he delivered a fiery speech from a makeshift stage in which he denounced the military’s removal of Morsi. His arrest followed the killing of his son Ammar, who was shot dead during violent clashes between security forces and Morsi supporters in Cairo on Friday.

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.