Egypt's Muslim Brotherhood and the opposition Rebel campaign have blamed each other for starting clashes in Alexandria on Wednesday night.
The violence began in the city's Raml district where Rebel campaigners were collecting signatures for an anti-Morsi petition.
Both sides exchanged volleys of stones and several people were injured. Gunshots were fired into the air by the Brotherhood side, an eyewitness said.
Al-Ahram Arabic news website said thousands of angry local residents chanted against the Brotherhood and besieged a mosque where its members had taken shelter. Police arrived and cordoned off the mosque.
Brotherhood spokesperson Anas El-Kady said its members were assaulted by Rebel campaigners. He claimed it was an early "start to violence," in reference to speculation that opposition protests planned for 30 June, the anniversary of the president's inauguration, could turn violent.
Rebel leaders have stressed the 30 June protests will be peaceful and have called on the interior ministry to protect demonstrators.
Rebel leading member Khaled El-Kady claimed the clashes on Wednesday began when its members were attacked by Islamists using bludgeons and bladed weapons.
Brotherhood members smashed projector screens where 'Liars under the name of religion' was to be screened, Khaled El-Kady told Al-Ahram Arabic News Website.
Anas El-Kady claimed six Brotherhood members were injured in the clashes.
Six Rebel members along with five local residents were injured, Khaled El-Kady said. Local residents saved their lives, he added.
Khaled El-Kady said there were less than ten Rebel campaigners at the time of the attack and dismissed claims by the Islamist group that they provoked the violence.
"The Brotherhood has been patient with such groups of thugs, abiding by its peaceful principles and refusing to be dragged into violence by the so-called civil, democratic groups," Anas El-Kady said, citing the opposition National Salvation Front, Black Bloc and Rebel.
Anas El-Kady claimed Rebel had been engaged in violence before under the name of Black Bloc, an anti-government youth group.
Rebel aims to collect 15 million signatures in support of a vote of no confidence in President Morsi, to outnumber the 13.2 votes he won in last year's presidential election. It had collected more than seven million signatures by late May.
Members of Rebel have been subject to intimidation, physical assaults, brief arrests and abductions since the group's inception. Last Friday, unknown assailants set fire to its headquarters in downtown Cairo.
Supporters of President Morsi say the Rebel campaign is anti-democratic as it attempts to prevent the elected president from finishing his 4-year term.