Clashes have erupted in Cairo's Tahrir Square as supporters and opponents of Egyptian President Mohamed Morsi tried to wrest control of the iconic square in rival rallies.
The state news agency on Friday cited a doctor at a hospital near Tahrir saying at least 100 people have been injured as protesters showered stones at each other in some of the worst violence over the country's new leader.
The clashes started after Brotherhood supporters tore down a podium belonging to a group that was chanting anti-Morsi slogans, witnesses said.
Morsi's supporters, mainly his powerful Muslim Brotherhood movement, had called their rally to denounce this week's acquittals of Hosni Mubarak-era officials.
His opponents, a coalition of liberal and secular leaning groups, had previously called their own rally to denounce Islamist control over a body drafting the new constitution, and Morsi's performance in office.
"Down with the Supreme Guide's rule," Morsi's opponents chanted, referring to Muslim Brotherhood leader Mohammed Badie.
Confusion reigned in the large square, the nerve centre of protests that toppled Mubarak early last year, as fighting broke out in several areas of the central Cairo hub.
Protesters torched two buses used by the Brotherhood to drive supporters into central Cairo, sending plumes of smoke into the air, witnesses said.
Al Jazeera's Sherine Tadros reporting from Cairo said that the clashes had intensified.
"The clashes were taking place just off Tahrir Square and on the side streets ... they are now happening right in the centre of the square," our correspondent said.
Despite multiple statements from Brotherhood leaders saying they would attend the rally, the group denied on Twitter that any of its members was involved in the fighting.
"We are not involved in Tahrir clashes, and none of our members were there," it wrote on its Twitter account, prompting a wave of derision from other people who posted videos of apparent Brotherhood members in Tahrir.
Morsi barely scraped through to win an election in June that presented voters with an unpopular choice between an Islamist president or Ahmed Shafiq, Mubarak's last premier.
Opponents of Morsi include influential judges who were again infuriated by the president when he tried sacking the state prosecutor on Thursday after the acquittals of 24 former regime figures accused of organising attacks on protesters.
The state prosecutor refused to step down and take Morsi's offer of an ambassadorship in the Vatican, and an influential judges' club described Morsi's decree as an attack on judicial independence.
After his election, Morsi had tried to reverse a court order that dissolved the Islamist-dominated parliament, sparking a backlash from the judiciary.