Demonstrators initially gathered for "Accountability Friday," organized by more than 21 political groups to protest what they consider President Mohamed Morsi's failure to fulfill promises about democracy and social justice, Ahram Online reported.
Fighting broke out after supporters of Morsi's Muslim Brotherhood party protested a Cairo Criminal Court decision Wednesday -- to acquit officials in the regime of ousted President Hosni Mubarak of charges they were behind the February 2011 Battle of the Camel in which 21 people were killed and hundreds injured.
An emergency medical official in Tahrir Square told Ahram Online about 100 people had been hospitalized Friday, mostly for treatment of head injuries, and "hundreds more" sustained other injuries.
Molotov cocktails and rocks were thrown and some demonstrators were shot with pellets. Leftist activist Kamal Khalil said Muslim Brotherhood demonstrators had fired the pellets.
"Our members haven't attacked anyone, the people fighting are supporters of the president," Brotherhood spokesman Mahmoud Ghozlan said.
Mohamed Waked of the National Front for Justice and Democracy posted a message on Twitter saying supporters of Morsi destroyed a stage and sound system that had been set up by Hamdeen Sabbahi's Popular Current, Ahram Online reported.
The fighting subsided when Brotherhood supporters left the square, Ahram Online said. The violence had mainly subsided and demonstrators began vacating Tahrir Square around 8 p.m., bikyamasr.com reported.
The Union of Revolutionary Youth first called for demonstrations against Morsi's 100-day platform in September, Egypt Independent reported. The organization called for protests to demand the dissolution and reformation of the Constituent Assembly, which is drafting the country's constitution.
The youth union soon gained support of the National Association for Change, the Constitution Party, the Egyptian Democratic Social Party, the Tagammu Party and other political organizations, the Independent said.
Khalil told al-Masry al-Youm the Muslim Brotherhood's protests over the acquittals was an attempt to shift attention away from protests against the country's new constitution.
"The Brotherhood is the ruling authority now; they were the ones who called for a reconciliation with former regime symbols ... and now they are calling for retribution?" Khalil asked.