Four persons were wounded by buckshot on Friday after opposition activists marched on thousands of Islamists rallying outside the Supreme Court in central Cairo demanding judicial reform, an official said.
In the clashes, the two sides pelted each other with stones and Molotov cocktails and gunfire was heard, according to an AFP journalist.
The correspondent added that opposition activists from the so-called Black Bloc, wearing black masks, fired buckshot at Islamists on the October 6 Bridge over the Nile River near Tahrir Square and wounded two people.
The head of the Egyptian rescue agency, Mohammed Sultan, told television that four people had been hospitalised with buckshot wounds.
An hour after the clashes broke out, three armoured police vehicles arrived and began firing tear gas, the correspondent said.
The Islamists, who set fire to two buses, were demanding an overhaul of the judiciary, after a court challenged a decision by Islamist President Mohamed Morsi to sack the veteran state prosecutor.
"The people demand the cleaning up of the judiciary," the protesters chanted.
Morsi's Muslim Brotherhood had called the demonstration outside the Supreme Court, which has repeatedly challenged Morsi since he took office last June.
Last month, a court overturned a controversial decree by Morsi to sack state prosecutor Abdel Meguid Mahmud, appointed by ousted president Hosni Mubarak, and replace him with Talaat Abdallah.
The court believed Morsi had overstepped his powers when he sacked Mahmud, blamed for bungling the trials of former regime officials, including Mubarak himself, after the 2011 uprising.
Many judges are Mubarak-era appointees, and Morsi supporters claim they remain hostile to them despite subsequent election victories.
A court also overturned Morsi's calling of parliamentary polls for this month, ruling that he had ratified a new electoral law without consulting the constitutional court.