Thousands of Islamists have clashed with their opponents in Egypt's second largest city, Alexandria, a day before the second leg of voting on a proposed constitution that has split the nation.

Riot police swung batons and fired volleys of tear gas to separate the stone-throwing crowds, made up of Muslim Brotherhood members and ultra-conservative Salafis on one side, and youthful protesters on the other.

The clashes started when the two groups met just after Friday afternoon prayers near the city's main mosque.

The demonstrators, some of whom carried black Islamic battle flags, withdrew from the mosque area under a heavy cloud of tear gas some two hours after the clashes began. Fighting continued along the coastal road of the Mediterranean city, near the Medical School and famed Bibliotheca Alexandrina.

At least 42 people were being treated for injuries, with some rushed to the hospital, an official said.

It was unclear who started the fight. Islamists had called for a big rally outside the Qaed Ibrahim mosque, and some 20 liberal political parties had said they would not hold a rival gathering to avoid clashes.

Security forces had cordoned off streets leading to the mosque as throngs of Salafi Islamists, most wearing the long beards favoured by the movement, gathered for what they called "the million-man rally to defend clerics and mosques." Some chanted "God is Great," and warned opponents: "with blood and soul, we redeem Islam."

The rally was called in response to violence last week, when a well-known Alexandrian Salafi cleric, Sheik Ahmed el-Mahalawi, was trapped inside a mosque for 12 hours while his supporters battled rock-throwing opponents outside with swords and firebombs.

El-Mahalawi, 87, had stirred anger with a sermon in which he denounced opponents of the Islamist-friendly draft charter as "followers of heretics." He denied that in a sermon today, accusing the media of spreading "lies," and claiming that last week's clashes were meant to prevent voting on the constitutional referendum.

The final round of voting on the disputed charter is to be completed tomorrow in the remaining 17 of Egypt's 27 provinces.

Critics charge that the Islamist-dominated body that wrote the draft document did not represent all Egyptians. Liberal and Christian members quit the assembly to protest clauses and articles they say were rammed through by hardline members aiming to create a religious state.

The opposition National Salvation Front reiterated its call today for voters to oppose the document, and one of the group's leaders, Mohamed ElBaradei, urged President Mohamed Morsi to suspend the referendum and form a new constituent assembly.

The first round of voting was held in 10 provinces last Saturday, including in Egypt's biggest cities, Cairo and Alexandria. Turnout was low, around 32%, and unofficial results showed around 56% of voters cast a "yes" vote in support of the constitution. Rights groups and the opposition immediately filed complaints alleging irregularities.

Controversy over the proposed constitution has in the past month plunged Egypt into political turmoil unprecedented since the February 2011 removal of Hosni Mubarak, the long-time authoritarian and secular-minded ruler.

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