At least 82 people were hurt on Friday after Egyptian opposition activists marched on thousands of Islamists rallying outside the Supreme Court in central Cairo demanding judicial reform, an official said.
The fighting took place in Abdel Moneim Square and on the October 6 Bridge that passes over it after crossing the Nile River. Islamists on the bridge threw rocks at militants below, including masked members of the so-called Black Bloc.
An AFP correspondent said some of the Black Bloc members fired birdshot at people on the bridge, wounding two of them.
Opposition activists also set fire to an empty bus that had brought Islamists to the rally.
The head of the Egyptian emergency services, Mohammed Sultan, told television at least 82 people had been injured.
An hour after the clashes broke out, three armoured police vehicles arrived and began firing tear gas, the correspondent said.
Prime Minister Hisham Qandil issued a statement warning that "demonstrations accompanied by violence completely harms the security and economy of the country and hampers plans for reform."
Morsi's presidency has been plagued by deadly clashes between protesters and police, a revolt in Suez Canal cities, sectarian violence and a devastating economic crisis, in what many fear is bringing Egypt to the brink of chaos.
Egypt is negotiating with the International Monetary Fund on a multi-billion dollar loan to stabilise its finances. That would require considerable reforms, and clinching it has been held up in part by the inability of the government to build a political consensus around the programme.
For its part, the interior ministry called on all political forces to aid law enforcement in "providing security to demonstrators to prevent confrontations resulting in victims."
The Islamists are demanding an overhaul of the judiciary, after a court challenged a decision by Islamist President Mohamed Morsi to sack the veteran state prosecutor.
Morsi's Muslim Brotherhood had called the demonstration outside the Supreme Court, which has repeatedly challenged the president since he took office in June.
Last month, a court overturned a controversial Morsi decree to sack state prosecutor Abdel Meguid Mahmud, appointed by ousted president Hosni Mubarak.
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.
A court also overturned Morsi's calling of parliamentary elections for this month, ruling that he had ratified a new electoral law without consulting the constitutional court.
Many judges are Mubarak-era appointees, and Morsi supporters claim they remain hostile to them despite subsequent election victories.
The Islamist-controlled Senate is currently preparing to debate legislation that would lower the retirement age for judges from 70 to 60, which many judges consider a manoeuvre to get rid of suspected anti-Morsi figures.