Opponents of Egypt's president's have scuffled with his supporters during a demonstration that posed the first test of Mohamed Morsi's popularity on the street.
Interior ministry sources told Al Jazeera that at 14 people were injured on Friday in skirmishes and thousands of protesters gathered in several areas of Cairo and Alexandria.
Opposition groups had called for a million-man march over their fears of what they say is the increasingly strong control of the Muslim Brotherhood over the country's politics.
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In Cairo's Tahrir Square, rival groups hurled stones at each other while some wielded sticks and charged their opponents.
The Muslim Brotherhood had asked its young followers to come out on Friday to "protect" its offices from opposition protesters, escalating concerns of a possible showdown in the capital.
The call for protests on Friday has spurred public debate especially after a Brotherhood cleric issued a religious edict, known as a fatwa, saying that killing anti-Islamist protesters was permissible.
The protest organisers demanded that Morsi repeal an interim constitution announced earlier this month in which he took over the military's powers to legislate in the absence of parliament, which the army had dissolved shortly before his election.
Morsi sacked military chief Field Marshal Hussein Tantawi and other senior army officials who had ruled the country after Mubarak's overthrow on August 12.
Wafaa Saad, the general co-ordinator of the "I am Egypt" campaign, said her group demanded that "the authorities protect the protesters, especially after some extremist clerics said these protesters [were] non-believers and that it [was] all right to kill them just because we [opposed] their opinion".
Scenes were calmer in other areas of the city where Morsi's opponents also gathered. But total numbers across the city were still relatively small by early afternoon, numbering in the hundreds.
Bashir Abdel Fattah, a political analyst and editor-in-chief of Democracy magazine, said "those demanding to burn the headquarters of the Muslim Brotherhood are extremists and that could lead to a wave of violence".
A spokesman for Morsi said that the president supported the right to stage protests and that "it is unhealthy" to spread fears about protesters' safety.
Security authorities said in a statement that they would "confront with all firmness ... riots or chaos that harms citizens' interests".
Several liberal groups usually critical of the Brotherhood stayed away from the protest, including the April 6 youth movement that helped galvanise support to oust President Hosni Mubarak in 2011.
Those declining to demonstrate did so either because they felt it was too early to judge Morsi, two months into his presidency, or that any challenge should be by ballot box and not on the street.
"Wake up Egyptian people. Don't fall for the Brotherhood," said Mahmoud, in his 50s, addressing about 200 people in Tahrir Square. "Egypt is for all Egyptians, not only one group."