Clashes have erupted between hundreds of protesters in Tahrir Square after pro-Brotherhood supporters destroyed the stage of Hamdeen Sabbahi's Popular Current.
Ahram Online's reporter on the ground says a number of people have been injured so far.
Supporters of President Mohamed Morsi currently make up the majority of protesters in the square as numbers continue to grow after Friday prayers.
Chants condemning the judiciary's decision to clear 24 former regime officials of orchestrating the Battle of the Camel are ringing out around the square.
Dozens of protesters are on their way from Cairo University on a march organised by Hamdeen Sabbahi's Popular Current to demand social justice, an independent judiciary and a constitution that represents all Egyptians.
A second march containing members of April 6 Youth Movement (Democratic Front), Revolutionary Socialists and Workers' and Peasants' Party is moving towards the square from Mustafa Mahmoud Mosque in Mohandiseen to call for a reformed Constituent Assembly.
Other marches with similar demands are on their way to the square from the Al-Nour, Al-Istikama, Sayeda Zeinab mosques and Dawran Shubra Square.
Fighting had broken out earlier in the day between pro- and anti-Muslim Brotherhood protesters in the square.
According to an Ahram Online reporter on the ground, pro-Brotherhood supporters attacked a stage built by secular groups and burnt a banner calling for the reformation of the Constituent Assembly.
There was stone-throwing in the nearby Mohamed Mahmoud Street around midday.
Two major protests are scheduled to take place in the square after Friday prayers.
Liberal and leftists forces have called for a protest dubbed "Accountability Friday" to mark the end of President Mohamed Morsi's first 100-days in office and to demand a reformed Constituent Assembly that represents all Egyptians. They are also calling for the implementation of a minimum and maximum wage and a reduction in the price of basic commodities.
The Muslim Brotherhood and other Islamists forces have called for a simultaneous protest, also in Tahrir Square, to condemn the acquittal of 25 former regime figures who were accused of involvement in the Battle of the Camel during the January 25 Revolution.