CAIRO—Hundreds of thousands of Egyptians rallied Friday to celebrate former President Hosni Mubarak's ouster a week earlier and remind the ruling generals that protest organizers can still muster daunting crowds if the military stalls on democratic reforms.

One of Islam's most influential and controversial clerics, Yussuf al-Qaradawi—whose popularity and defiance of the regime got him banned from entering Egypt under Mr. Mubarak—made his comeback onto the Egyptian political stage to deliver the traditional Friday sermon to worshippers in Cairo's Tahrir Square, which protestors occupied throughout most of the 18-day uprising.

Mr. al-Qaradawi, who has close ties to the Muslim Brotherhood, returned to Egypt Thursday night from Qatar, where he spent years in exile. He is the latest example of how a diverse array of political and religious leaders are energizing Egypt's long-stagnant political scene. Old regime opponents whom Mr. Mubarak had once kept in check, such as Mr. al-Qaradawi, now appear to be forging warmer ties with the military. The Muslim Brotherhood, a once-banned Islamist group, has been a major beneficiary of the more open political landscape and now plays a key role in the military-led democratic transition.

Protest leaders, who had called for Egyptians to come out Friday to celebrate as well as to commemorate those killed in the uprising, said they hoped Friday's gathering would serve up a pointed reminder to the military, which has ruled the country since Mr. Mubarak stepped down on Feb. 11, that the same masses who ousted Mr. Mubarak could marshal their forces again if the military regime failed to press ahead with promised reforms.

The military has moved cautiously to respond to protesters' demands. It dissolved parliament, vowed to hold elections within six months, and suspended the constitution. But Egypt's draconian emergency law, which gives the government broad powers to restrict civil liberties, remains in place. The military continues to hold those arrested during the uprising, and some have been tortured, according to human-rights officials. Thousands of other political prisoners also have yet to be released.