Egyptian police fired tear gas to prevent opponents of President Mohamed Mursi storming a court and the prosecutor-general's office in central Cairo on Saturday, witnesses said.
Eight people were injured in Cairo during another protest in al-Fayoum south of the capital, the state news agency MENA said.
Some 500 people marched for much of the day through central Cairo, chanting "The people want to topple the regime" on the fifth anniversary of the founding of the opposition April 6 youth movement.
When some protesters hurled fireworks and rockets at the court, which also houses the prosecutor's office, and tried to break down the main gate, police fired tear gas from upstairs windows, witnesses said.
Egypt has been in political turmoil since the ousting of President Hosni Mubarak in 2011, with Mursi and his Islamist allies pitted against various secular opposition groups.
The prosecutor-general angered activists a week ago by questioning a popular TV satirist who is accused of having insulted Mursi. The government denies opposition claims that the case is evidence of a crackdown on dissent.
"We are Muslims but we want a civilian state," said Saffeya Mustapha, a female protester on the Cairo march.
Seven protesters were taken to hospital, MENA said, citing the Health Ministry.
Separately, opponents and supporters of the Muslim Brotherhood, Mursi's Islamist allies, attacked each other with stones and fireworks in the Mediterranean city of Alexandria, MENA said. It did not report any injuries.