ALEXANDRIA, Egypt — Thousands of Islamists clashed with their opponents on Friday in Egypt’s second largest city, Alexandria, a day before the second leg of voting on a proposed Islamist-backed constitution that has polarized the nation.

Meanwhile, the country’s Islamist President Mohammed Morsi made a last-minute move to tighten his grip on power by appointing 90 members to parliament’s upper house, a body set to wield temporary lawmaking powers if the constitution is approved by referendum.

In Alexandria, on the Mediterranean coast, riot police swung batons and fired volleys of tear gas to separate stone-throwing Muslim Brotherhood members and ultraconservative Salafis on one side, and youthful protesters on the other. The clashes started when the two groups met just after Friday afternoon prayers at the city’s main mosque near the coastal road.

Witnesses say youth set fire to four vehicles — two buses and two cars — belonging to Islamists, sending thick black smoke through the upscale city center. The demonstrators, some of whom carried black Islamic battle flags, withdrew under a heavy cloud of tear gas some two hours after the clashes began. Fighting continued into dusk along the corniche, near the Medical School and famed Alexandria Library.

At least 42 people were treated for injuries, with some rushed to the hospital, a city health official said on condition of anonymity because he was not authorized to brief reporters.

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 favored 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 on Friday, 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 Saturday. 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 on Friday for voters to oppose the document, and one of the group’s leaders, Mohamed ElBaradei, urged Morsi to suspend the referendum and form a new constituent assembly.

“If this constitution passed, there will be no stability,” said Baradei, a Nobel Laureate and Egypt’s leading pro-democracy advocate.

With election authorities, army and police preparing for Saturday’s voting however, ElBaradei’s televised message looked unlikely to shift Morsi’s position.

The first round of voting was held in 10 of Egypt’s 27 provinces last Saturday, including the biggest cities — Cairo and Alexandria. Turnout was low, around 32 percent, and unofficial results showed around 56 percent of voters cast a “yes” vote in support of the constitution. Rights groups and the opposition immediately filed complaints alleging irregularities.

Controversy surrounding proposed constitution has in the past month plunged Egypt into its worst turmoil since the February 2011 ouster of Hosni Mubarak, the longtime authoritarian and secular-minded ruler.

The draft has split the country into two camps. On one side are the Islamists from the country’s most organized political group, the Muslim Brotherhood, from which President Mohammed Morsi hails, and their backers from various Salafi and former Jihadi groups.

The opposition camp, led by the National Salvation Front, is an alliance of liberal parties and youth groups backed by Christians and moderate Muslims who fear the Brotherhood is attempting to monopolize power by passing a constitution that enshrines a greater role for clerics and Islamic law.

Hundreds of thousands of Egyptians from both sides have rallied in the streets over the past month. The crisis peaked when Brotherhood supporters attacked an opposition sit-in outside the presidential palace in Cairo on Dec. 5. The ensuing violence left at least 10 dead and hundreds of injured on both sides.

The crisis was compounded by Morsi’s decision to rush the draft constitution to a referendum after an Islamist-dominated panel approved it, as well as his move last month to grant himself near-absolute powers, which were later rescinded.

Morsi’s moves have also split state institutions. The judiciary became another battleground, with the powerful Judges’ Club calling on its members to boycott the constitution vote while Brotherhood sympathizers in the legal system and other independents insisted on supervising it.

Egyptian prosecutors held a sit-in protest to press Morsi-appointed prosecutor general Talaat Abdullah to resign on Monday. Abdullah resigned, then retracted his resignation on Thursday, raising the prospect of new protests by fellow prosecutors.

Also, Zaghloul el-Balshi, the secretary general of the election committee who is also a judge and an aid to the country’s justice minister, resigned Wednesday, citing health reasons. The media said his resignation was prompted by his inability to prevent voting violations in the first leg of the referendum.

Morsi’s move to pick appointees for parliament’s upper house came ahead of a deadline — the constitution, if passed, would limit presidential appointees to only 10. The 270-member Shura Council, normally purely an advisory body, would hold legislative authority until the lower house is elected two months after the constitution passes. Most seats are already held by Islamists.