Egyptians decide on Saturday on a constitution promoted by its Islamist backers as the way out of a prolonged political crisis and rejected by opponents as a recipe for further divisions in the Arab world's biggest nation.Voting begins in a referendum on a divisive draft basic law that has pitted Islamist supporters of President Mohamed Morsi against a liberal, secular and Christian opposition in often bloody clashes in Cairo and other cities.The opposition says the constitution is too Islamist and tramples on minority rights. Morsi's supporters say the charter is needed if progress is to be made towards democracy nearly two years after the fall of military strongman Hosni Mubarak.Mere hours after the polls opened on Saturday, a man was arrested for shooting in the air outside a polling station in Gharbiya in the first sign of violence since the voting started, Al-Masry Al-Youm reported.Egyptian opposition, the National Salvation Front, reportedly documented over 50 violations at voting sites across the country. The opposition claimed that a judge overseeing voting in the referendum directed voters to vote "yes" on the constitution, Al-Masry Al-Youm reported.Other reports indicated that supporters of the Muslim Brotherhood were campaigning in the vicinity of the polling stations by distributing flyers promoting voting "yes" on the constitution, according to Al-Masry Al-Youm.In Alexandria on Friday, tensions boiled over into a street brawl between rival factions armed with clubs, knives and swords. Several cars were set on fire and a Muslim preacher who had urged people to vote "yes" to the constitution was trapped inside his mosque by angry opposition supporters. In the capital, Cairo, both sides made low-key final efforts to rally supporters.Flag-waving Islamists gathered peacefully at one of the main mosques, some shouting "Islam, Islam" and "We've come here to say 'yes' to the constitution".Opposition supporters - who have been urged to vote "no" by their leaders - assembled outside the presidential palace.The building remains ringed with police, soldiers and tanks after street clashes caused at least eight deaths earlier this month in violence prompted by Morsi's decision to award himself sweeping powers in order to ram through the new charter.As darkness fell, there appeared to be more bystanders and street vendors present than opposition demonstrators. A woman addressed the crowd through a loudhailer, shouting obscenities about Morsi, but many in her audience seemed more interested in drinking tea or having their picture taken in front of a tank."I don't like Morsi," said Moustafa Ahmed, 25, a teacher. "But I haven't decided what to vote for tomorrow so I decided to come here to listen to the protesters and chat to them one last time."Two days of votingThe referendum will be held on two days - this Saturday and next - because there are not enough judges willing to monitor all polling stations after some in the judiciary said they would boycott the vote.Egyptians are being asked to accept or reject a constitution that must be in place before a parliamentary election can be held next year - an event many hope can steer the country towards stability.The measure is generally expected to pass, given the well-organized Muslim Brotherhood's record of winning elections since the fall of Mubarak. Many Egyptians, tired of turmoil, may simply fall in line and vote "yes".If the constitution is voted down, a new assembly will have to be formed to draft a revised version, a process that could take up to nine months.Just over half of Egypt's electorate of 51 million will vote in the first round in Cairo and other cities. Polling stations open at 8 a.m. and close 12 hours later.Official results will not be announced until after the second round, though it is likely that details will emerge after the first round that will give an idea of the overall trend.To provide security for the vote, the army has deployed about 120,000 troops and 6,000 tanks and armored vehicles to protect polling stations and other government buildingsWhile the military backed Mubarak and his predecessors, it has not intervened on either side in the present crisis.The charter has been criticized by some overseas bodies.The International Council of Jurists, a Geneva-based human rights group, said it falls short of international standards on the accountability of the armed forces, the independence of the judiciary, and recognition of human rights.United Nations human rights experts said the draft should be reviewed to ensure that Egypt meets its obligations under international law on equality and women's rights.