The demonstration ended an uneasy truce between Egypt’s Islamists and its military, which had prevailed since Mr. Mubarak’s exit. The truce reached a breaking point after the military council spelled out for the first time its intention to claim a decisive role in Egyptian politics far into the future, even after parliamentary elections scheduled to begin later this month or a final handover of power to constitutional authorities some time in 2013 or beyond.

It begins a faceoff between Egypt’s two most powerful institutions, its army and the once-outlawed Muslim Brotherhood, that left Egyptian liberals anxious and divided on the sidelines…

Egyptian liberals, torn between their fears of Islamist power on the one hand and of military rule on the other, mostly stayed home. The April 6 Movement, a pivotal force in the uprising, was one of the few liberal groups to make a conspicuous presence, calling it “the Friday of One Demand”—meaning a handover of power to the lower house of parliament after it is elected by April. “Of course there are fears of Islamists taking power,” said Dina Allithy, 23, a recent college graduate and member of the group. “But today we are trying to ignore all of that.”