CAIRO—Mobs of ordinary Egyptians joined with soldiers to drive pro-democracy protesters from their encampment in Tahrir Square here Monday, showing how far the uprising's early heroes have fallen in the eyes of the public.

Six months after young, liberal activists helped lead the popular movement that ousted President Hosni Mubarak, the hard core of these protesters was forcibly dispersed by the troops. Some Egyptians lined the street to applaud the army. Others ganged up on the activists as they retreated from the square that has come to symbolize the Arab Spring.

Squeezed between an assertive military and the country's resurgent Islamist movement, many Internet-savvy, pro-democracy activists are finding it increasingly hard to remain relevant in a post-revolutionary Egypt that is struggling to overcome an economic crisis and restore law and order.

"The liberal and leftist groups that were at the forefront of the revolution have lost touch with the Egyptian people," says Shadi Hamid, director of research at the Brookings Institution's Doha Center. "These protesters have alienated much of Egypt. For some time they've been deceiving themselves by saying that the silent majority is on their side—but all evidence points to the contrary, and Monday's events confirm that."

Monday's turmoil in Tahrir followed a massive Friday demonstration on the same square by hundreds of thousands of Islamists, who called for transforming Egypt into an Islamic state—and railed against the liberal and secular youths who had helped motivate millions to rise up against Mr. Mubarak.