Summary Clashes rival groups broke out in the Egyptian city of Alexandria on the eve of referendum.
Stone-throwing clashes broke out Friday in the Egyptian city of Alexandria between Islamists and opposition protesters, on the eve of a highly charged referendum on a new constitution, witnesses and state media said.
Fifteen people were wounded, according to medics, and several cars set on fire in the violence in the Mediterranean city, Egypt s second largest.
State television showed at least one protester brandishing a sword.
Riot police quickly moved to separate the protesters, who clashed after a cleric urged worshippers to vote yes for the constitution. Hundreds of protesters kept up a siege of the mosque in which the cleric took refuge.
Tensions are high over the staggered referendum, which is being held on this and the following Saturday, after weeks of protests and violence between rival camps in Cairo that killed eight people and injured hundreds last week.
Further rallies by both sides were taking place in Cairo on Friday.
A pro-constitution rally by the Muslim Brotherhood backing Islamist President Mohamed Morsi gathered more than 2,000 people, and separately, hundreds of Morsi s opponents demonstrated outside his palace.
Polling in the referendum was staggered over a week because of a shortage of judges willing to provide the statutory supervision for the vote.
Voting will begin on Saturday in Cairo and Alexandria and in eight other provinces. The other half of the country will vote on December 22.
International watchdogs, including the UN human rights chief, and the United States and the European Union, have expressed reservations about the draft because of loopholes that could be used to weaken human rights, the rights of women and the independence of judges.
Analysts said the proven ability of the Muslim Brotherhood to muster voters was likely -- but not certain -- to ensure that the draft constitution is passed.
If it is adopted, "it will exacerbate political tension and result in more acrimony," Hani Sabra, an Egypt specialist for the Eurasia Group, said in an evaluation note.
"The Brotherhood and Morsi believe that if the constitution is adopted, it represents a mandate to pursue their policies.
However, majority support does not translate into the bureaucracy falling in line. It will also not silence the opposition."
Polls open at 8:00 am (0600 GMT) and are scheduled to close at 7:00 pm (1700 GMT).
