FAYOUM, Egypt — They hide in safe houses on the outskirts of this city, talk only fleetingly on cellphones and avoid the cafes where they used to meet. Heavy scarves obscure their identities when they venture out to join protests.

The leaders of the Muslim Brotherhood, now outlawed, have adjusted to life underground, even while hundreds of their fellow members have been arrested in this city since the military deposed President Mohamed Morsi, a Brotherhood leader, and the Egyptian government branded the group a terrorist movement.

Yet, rather than crack and disintegrate under the pressure, members say, the group has fallen back on the organizational structure that sustained it for decades as a banned and secretive movement. It is becoming more decentralized, but also more cohesive and rigid, as its members abandon activities like preaching and social work and shift their attention to a virtually singular goal: resistance to the military-backed government.

Their focus, many Brotherhood members say, is a protracted, grinding struggle.

“There is a vision of a political confrontation that can go on for years,” said one leader, a 33-year-old architect in Fayoum, an Islamist stronghold. He, along with other members, spoke in a cafe for a time, but changed locations after suspecting the employees were eavesdropping.