CAIRO—The Egyptian opposition's takeover of the area around the parliament this week began with a trick—the latest example of how, for more than two weeks, young activists have outwitted Egypt's feared security forces to spur an uprising many here had long thought impossible.

On Tuesday, young opposition organizers called for a march on the state television building a few blocks north of their encampment in central Tahrir Square. Then, while the army deployed to that sensitive communications hub, protesters expanded southward...