Tahrir Square in Cairo filled with people on Friday after the Muslim Brotherhood called for nationwide demonstrations.
The event was the biggest since President Hosni Mubarak stepped down on 11 February.
Many called for an Islamic state and Sharia law.
The Brotherhood is the most organised political force in Egypt, although it was not prominent in the revolution.
Protesters have camped in Tahrir Square since early July.
Liberal groups first want guarantees of a constitution that will protect religious freedom and personal rights, whereas Islamists want speedy elections and a recognition of Islam in the new state.
Now, the Islamists want their voice to be heard. A BBC correspondent says this development could mark a turning point in the post-revolution period.