Dozens of activists fought with clubs and swords, witnesses said, and a number of cars were set alight on the streets of Egypt's second biggest city on the eve of a vote that Mursi hopes will bring an end to the country's worsening political crisis.
Scuffles started near a mosque in Alexandria when opposition members handing out flyers clashed with Mursi supporters.
In Cairo, flag-waving pro-Mursi Islamists staged a final rally on Friday before the referendum, but the gathering outside one of the capital's main mosques was peaceful.
Cairo and other cities have seen often violent demonstrations over the past three weeks since Mursi assumed sweeping new powers to push through the constitution, which he sees as a vital element of Egypt's transition to democracy after the overthrow of autocratic predecessor Hosni Mubarak last year.
At least eight people have died and hundreds have been injured, and a leading opposition figure warned of more blood on the streets during the voting this Saturday and next on a draft the opposition says is too heavily influenced by Islamists.
The referendum, held on two days because there aren't enough judges willing to monitor all polling stations, asks Egyptians to accept or reject a basic law that must be in place before national elections can be held early next year - an event many hope can steer the country towards stability. The government said Saturday would be a holiday to enable people to vote.
To bolster support for the constitution, Islamists who propelled Mursi to power in June's presidential election assembled at a mosque near the president's palace in Cairo.
"Islam, Islam," they chanted. "We've come here to say 'yes' to the constitution."
The majority of protesters were men with beards, and some had brought their children and veiled wives with them.
Members of the liberal, secular and Christian opposition began to gather outside the presidential palace to protest against the basic law.
Mohamed ElBaradei, an opposition leader and Nobel prize winner, issued a statement urging Mursi to cancel the referendum "before it is too late".
Amr Moussa, a former head of the Arab League also prominent in the opposition, called on Egyptians to vote "no".
The measure is nevertheless 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.Solid indication
The first round of voting on Saturday will take place in Cairo and other major cities. Official results won't be announced until after the second round, though it is likely that details will emerge to give a good steer on the first-day figures, which are expected to show a strong vote in favour.
To provide security for the vote, the army has been deployed in force, with state television showing ranks of soldiers receiving their orders to protect polling stations and other government buildings. About 120,000 troops and 6,000 tanks and armored vehicles will be deployed.
While the military backed Mubarak and his predecessors, it has not intervened on either side in the present crisis.
The opposition says the constitution does not reflect the aspirations of all 83 million Egyptians because it is too Islamist and tramples on minority rights, including those of the Christian community. Mursi's supporters say the constitution is needed if progress is to be made towards democracy.
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.
While the opposition is telling its supporters to vote "no", it has also threatened to boycott proceedings if guarantees for a fair vote are not met. But staying away from the process could risk a loss of credibility, political experts say.
Many ordinary Egyptians are well aware of how contentious the constitution is, but simply want to get it out of the way so the country can move ahead.
"I am so tired of politics," said Ahmed Shawki, who works in downtown Cairo. "Can't we have a referendum on stability instead of the constitution and then all Egyptians work together to achieve stability. A faraway dream."
CAIRO (Reuters) - Stone-throwing supporters and opponents of President Muhammad Mursi clashed in the Egyptian city of Alexandria on Friday ahead of a referendum on a new constitution that has divided the Arab world's most populous nation.