Egyptian police fired tear gas on Friday to disperse clashing Islamists and opposition protesters in the Egyptian city of Alexandria, on the eve of a highly charged referendum on a new constitution, witnesses said. At least fifteen people were wounded, medics said, and several cars set on fire in violence between opposition protesters and Islamists wielding machetes which began after a Muslim preacher called for support for the constitution. Protesters then laid siege to the mosque where the preacher sought refuge, and police fired tear gas to drive them away when the clashes with Islamists erupted again after nightfall, witnesses said. 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. The protests were modest in size compared with mass rallies in recent weeks by the opposition that forced Morsi to give up sweeping powers, and huge protests by Islamists to show that the president had supporters. Morsi\'s Muslim Brotherhood and ultra-orthodox Salafist groups backing the draft charter have been campaigning for days for Egypt\'s 51 million voters to approve it. A senior Islamist official involved in drafting the charter, Amr Darrag, branded opposition attacks against the document an \"unjust campaign to paint a lying picture of the constitution.\" Egypt\'s mainly secular opposition has criticised the draft charter as divisive and a possible attempt by Islamists to introduce sharia-style law. \"We are confident that the Egyptian people will topple the Muslim Brotherhood\'s constitution,\" Amr Hamzawy, a leader of the opposition National Salvation Front, told a news conference. \"The National Salvation Front calls on people to flood into polling stations to say no\" to the draft charter, another Front leader, Hamadeen Sabahi, said. The Front was holding last-ditch rallies on Friday against the new constitution. It has said it could yet call a boycott if its \"deep concern\" over the referendum\'s fairness turns out to be founded. The opposition took out half-page advertisements in major independent dailies describing the charter as \"a constitution that divides Egypt.\" The opposition -- which initially wanted the referendum postponed -- only started urging a \"no\" vote on Thursday. \"It\'s you who will pay the price if you vote \'yes.\' \'No\' to the constitution,\" said an online campaign advertisement by the opposition April 6 youth movement. Morsi has ordered Egypt\'s military to help police maintain security until the results of the referendum are known. A total of 130,000 police and 120,000 soldiers will be deployed, interior ministry and military officials told AFP. 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).