"The Brotherhood denounces violence and says it is committed to peaceful protest. But as members go into hiding, its key building blocks - local groups of seven members known as usras - are under pressure."
In Egypt's second city, medical student Ahmed Nabil lives in fear that the police may come and arrest him any day. As a member of the Muslim Brotherhood, he is part of a movement facing an onslaught by the security forces which toppled Islamist President Mohamed Mursi in July.
"These days we can be picked up at any time," said Nabil, whose parents are also members of the organisation, Egypt's oldest Islamist movement and a supporter of Mursi.
The Brotherhood's discipline and hierarchy helped it win elections after the 2011 popular uprising that ousted autocrat Hosni Mubarak, eventually propelling Mursi into power. But now the army-led government and its supporters regard the Brotherhood as a terrorist group and enemy of the state. The security forces and police, feared and despised under Mubarak, are lauded for cracking down on the organisation.