Supporters of Egypt's President Mohamed Morsi clashed with opponents in Cairo's Tahrir Square leaving more than 100 injured, officials said, in the worst violence over the Islamist leader.
Morsi's Muslim Brotherhood movement and a coalition of secular-leaning groups held separate rallies on some of the thorniest issues facing the new democracy after last year's uprising which ousted president Hosni Mubarak.
The health ministry said at least 110 people were injured as protesters showered each other with stones and petrol bombs, after Morsi supporters tore down a podium from which anti-Brotherhood chants were being orchestrated.
Protesters torched two buses used by the Brotherhood to drive supporters into central Cairo, sending plumes of smoke into the air, witnesses said.
The violence erupted as Morsi faced a backlash from judges after trying to sack the chief prosecutor following this week's acquittals of Mubarak-era officials on trial for a deadly attack on protesters during the 2011 uprising.
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 others who posted videos of apparent Brotherhood members in the square.
An AFP correspondent said the clashes subsided when Islamists withdrew from the square in the evening. State television reported that they held a protest outside the state prosecutor's office nearby.
In a speech in the coastal city of Alexandria, Morsi pledged to bring to justice officials accused of organising the killings of protesters during the uprising that eventually brought his once-banned movement to power.
"We will never ignore those who committed crimes against the nation and corrupted it," he said in the speech reported by the official MENA news agency.
But other groups that had taken part in the 18-day uprising and now oppose Morsi accuse the Islamists of dominating political life, particularly a crucial body that is drafting Egypt's new constitution.
"The Brotherhood realised today that the Muslim Brotherhood is not all of Egypt," said protester Khaled Abdel Ghaffar, an opponent of the movement.
Morsi narrowly won a June election which presented voters with an unpopular choice between an Islamist president and Ahmed Shafiq, Mubarak's last prime minister.
Friday's clashes were the most violent in a simmering struggle between Morsi's movement and his opponents, who range from Mubarak loyalists to groups which spearheaded the revolt against the former dictator.
He also faces intense opposition from powerful judges who accuse him of trying to detract from their authority.
An influential group of Egyptian judges backed state prosecutor Abdel Meguid Mahmud's refusal to resign after Morsi ordered his removal on Thursday, the official Al-Ahram newspaper reported.
Morsi's bid to remove Mahmud bypassed checks on presidential control of the prosecutor, further enraging judges after the president had unsuccessfully tried to reverse a court order disbanding the Islamist-dominated parliament.
Mahmud said he received threats from a senior judge and an ally of Morsi who told him he could be assaulted if he did not heed the decree to resign.
Ahmed al-Zind, head of the Judges' Club, said the judiciary was backing Mahmud to uphold "the sovereignty of the law and the principle of separation of powers," Al-Ahram reported.
He said the judges would hold an emergency meeting "to confront the current crisis that aims at harming the judiciary."
Morsi had pledged to retry Mubarak and his senior officials for their roles in the killing of protesters during the revolt, after trials that critics said had been bungled by the state prosecutor's office.
On Wednesday, a court acquitted 24 people -- including the former speakers of Egypt's two houses of parliament, Safwat al-Sherif and Fathi Surur -- of organising a camel-borne attack on anti-Mubarak protesters during the uprising.
The February 2, 2011, assault by pro-Mubarak supporters -- some riding horses and camels -- on protesters in Tahrir Square came on one of the revolt's bloodiest days, when clashes left more than 20 dead.
A total of around 850 people died during the 18-day uprising.