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 into the lightly defended area around Egypt's parliament building.

As Egypt's antigovernment protests reached their 17th day on Thursday, President Hosni Mubarak's regime was deep in turmoil. The head of the ruling National Democratic Party said he advised Mubarak to step down. The country's army moved to take control of the streets. But Mubarak, to the rage of demonstrators, didn't step aside.

The demonstrations that now bedevil Mubarak across Cairo and Egypt took seed in part thanks to one trick play, interviews with several protest planners show.

On Jan. 25, the first day of protests, the organizers from the youth wings of Egypt's opposition movements created what appeared to be a spontaneous massing of residents of the slum of Bulaq al-Dakrour, on Cairo's western edge. These demonstrators weren't, as the popular narrative has held, educated youth who learned about protests on the Internet. They were instead poor residents who filled a maze of muddy, narrow alleyways, massed in front of a neighborhood candy store and caught security forces flatfooted.

That protest was anything but spontaneous. How the organizers pulled it off, when so many past efforts had failed, has had people scratching their heads since.

After his release from detention Sunday, Google Inc. executive Wael Ghonim recounted his meeting with Egypt's newly appointed interior minister. "No one understood how you did it," Mr. Ghonim said the minister told him. He said his interrogators concluded that outside forces had to have been involved.

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