Egyptian officials say 28 people have been killed and more than 80 injured in clashes across the country between anti-government protesters and security forces.
The violence came as supporters of Egypt's military marked a national holiday for the 40th anniversary of the 1973 Arab-Israeli war.
Pro-military crowds and backers of ousted president Mohamed Morsi poured into streets, turning the holiday into a showdown between the rival camps.
The military-backed government had hoped to highlight popular support during the holiday for de facto leader General Abdel Fatah el-Sissi. But the Muslim Brotherhood organized its own marches under the banner "Reject the Coup."
Sunday's protests appeared to be the biggest turnout since the mid-August crackdown on supporters of Mr. Morsi. In the ensuing months, the protest movement has changed focus from a drive to reinstate the Islamist president to a wider anti-military stance.