At least 82 people were hurt on Friday in clashes after opposition activists marched on thousands of Islamists rallying outside a central Cairo court demanding judicial reform, an official said.
The fighting erupted near the iconic Tahrir Square, roughly 0.5 kilometres from where the Islamists had staged their rally, with both sides trading stones.
A few activists on the opposition side fired homemade guns loaded with birdshot at the Islamists, who had taken over a main bridge that crosses the Nile River.
Riot police on foot and in armoured vehicles succeeded by nightfall in creating a cordon between the two sides, but ended up clashing with the opposition activists.
A riot police vehicle on a side street came under fire from birdshot rifles as Islamists ducked for cover behind the armoured vehicle.
A police officer fired back what appeared to be birdshot from a rifle as Islamist protesters cheered, but an interior ministry official later insisted to AFP that police had been armed with only tear gas and blanks.
"The people demand the toppling of the regime," the opposition protesters chanted -- the signature slogan of the early 2011 uprising that ousted president Hosni Mubarak and eventually brought Islamists into power.
"Morsi! Morsi!" the Islamists chanted back, referring to President Mohamed Morsi, a former Muslim Brotherhood leader.
The head of the Egyptian emergency services, Mohammed Sultan, told television that at least 82 people had been hospitalised.
An interior ministry statement said police arrested 19 suspects in Cairo's clashes, including three young men suspected of torching a bus that had transported the Islamists to their rally.
Islamists and their opponents also clashed in the Mediterranean city of Alexandria, where more than a dozen people were injured by stone throwing, witnesses said.
Prime Minister Hisham Qandil issued a statement warning that "demonstrations accompanied by violence completely harm the security and economy of the country and hamper 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.
Since Morsi's election in June, the Islamist leader has sought to face down an increasingly vocal opposition that accuses him of betraying the goals of the 2011 uprising, and Morsi has even had to confront unprecedented strikes by the police.
In Friday's clashes, police had initially withdrawn after their first attempt to separate the protesters, thinking the Islamists would use them as shields, the police lieutenant colonel who ordered his conscripts to retreat told AFP.
They later massed again when the Islamist protesters agreed to hang back.
The Islamists rallying on Friday were demanding an overhaul of the judiciary, which they believe is inimical to Morsi.
Last month, a court overturned a controversial decree by Morsi to sack state prosecutor Abdel Meguid Mahmud, appointed by 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.
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.
A court had 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.
Friday's violence came days after a negotiating team from the International Monetary Fund left Egypt after talks over a key $4.8 billion loan that Morsi's government hopes will revive the badly hit economy.
Egypt is negotiating with the International Monetary Fund the 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.