CAIRO  Egypt’s powerful Muslim Brotherhood and the secular opposition banded together Sunday around a prominent government critic to negotiate for forces seeking the fall of President Hosni Mubarak, as the army struggled to hold a capital seized by fears of chaos and buoyed by euphoria that three decades of Mr. Mubarak’s rule may be coming to an end.

The announcement that the critic, Mohamed ElBaradei, would represent a loosely unified opposition reconfigured the struggle between Mr. Mubarak’s government and a six-day-old uprising bent on driving him and his party from power.

Though lacking deep support on his own, Dr. ElBaradei, a Nobel laureate and diplomat, could serve as a consensus figure for a movement that has struggled to articulate a program for a potential transition. It suggested, too, that the opposition was aware of the uprising’s image abroad, putting forth a candidate who might be more acceptable to the West than beloved in Egypt.

In scenes as tumultuous as any since the uprising began, Dr. ElBaradei defied a government curfew and joined thousands of protesters in Liberation Square, a downtown landmark that has become the epicenter of the uprising and a platform, writ small, for the frustrations, ambitions and resurgent pride of a generation claiming the country’s mantle.