Tens of thousands of people have gathered in Cairo's Tahrir Square after demonstrators called for an Islamic state and Sharia law in the country.
It is the first time Islamist leaders have called for nationwide demonstrations since President Hosni Mubarak was overthrown in February.
The demonstrators - dominated by Muslim Brotherhood supporters - will cause much concern for secularists as tensions remain high in the transition to democracy, BBC reports.
It was one of the largest demonstrations since Mubarak was pushed from power.
Yesterday, it was announced that Mubarak would go on trial, along with his two sons and eight other associates, at a Cairo convention center with hundreds of seats for an audience and heavy security.
He is due to go on trial on Wednesday, charged with killing hundreds of anti-government protestors, though his lawyer is saying he is too unwell to travel to the capital.
The Brotherhood is the most organized political force in Egypt, although it was not prominent in the revolution, it says.
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 one form or another - in the new Egyptian state.
Now, the Islamists want their voice to be heard and are showing their muscle for the first time since Mr Mubarak stepped down on 11 February, says the BBC's Jon Leyne in Cairo.
Although the Muslim Brotherhood - Egypt's oldest political party - can turn out huge crowds by rallying its supporters at mosques, it does not necessarily represent the majority of Egyptians and is predicted to win around 20% of the vote in an election, Leyne says.
There was little sign of any secular groups at Friday's rally, he says, adding that it will be interesting to see how they re-group after today's events.
Since early July, the mainly secular protesters had camped out in Tahrir Square, angry with the slow progress towards reform, but Islamists had mostly stayed away.
Last week, they held their own demonstration and accused the Tahrir protesters of going against the country's "Islamic identity", the AFP news agency reports.
Leyne says the move by the Brotherhood could make a significant turning point.
On the podiums set up around the square, speakers called for unity and partnership, but inside crowds called for Egypt to "implement God's law." AFP reports.
Several hundred protesters stampeded out of the square, screaming and causing momentary panic, only to return later and find that nothing had happened, it says.
"Don't worry, people were just chanting Allahu Akbar (God is greatest)," said one demonstrator, explaining that the fervor of the chant may have scared people into thinking something had happened, AFP reports.