Clashes broke out on Thursday between supporters and opponents of deposed president Mohamed Morsi in the city of Zagazig, the capital of Egypt's Sharqiya governorate.
Protests had erupted earlier in the day, with thousands of local residents taking to the streets to demand Morsi's reinstatement.
Police fired teargas at pro-Morsi demonstrators, but this did little to disperse the crowds, Ahram Online correspondent Adel El-Shaer reported.
According to state news agency MENA, police were eventually able to disperse the protesters.
Egypt's health ministry reported that 80 had been injured by rubber bullets, rocks, batons and bladed weapons.
Fearing an outbreak of looting, most local shop owners closed their places of business.
Eleven people were reportedly arrested and charged with incitement to violence following the clashes.
Egypt has been plagued by political violence in recent days between supporters and opponents of the former president, who was ousted by Egypt's military on Wednesday amid mass demonstrations.