For decades, Egypt’s authoritarian president, Hosni Mubarak, played a clever game with his political opponents.

He tolerated a tiny and toothless opposition of liberal intellectuals whose vain electoral campaigns created the facade of a democratic process. And he demonized the outlawed Muslim Brotherhood as a group of violent extremists who posed a threat that he used to justify his police state.

But this enduring and, many here say, all too comfortable relationship was upended this week by the emergence of an unpredictable third force, the leaderless tens of thousands of young Egyptians who turned out to demand an end to Mr. Mubarak’s 30-year rule.

Now the older opponents are rushing to catch up.

“It was the young people who took the initiative and set the date and decided to go,” Mohamed ElBaradei, the former head of the International Atomic Energy Agency, said Wednesday with some surprise during a telephone interview from his office in Vienna, shortly before rushing home to Cairo to join the revolt.