Egyptian police have fired tear gas as supporters of President Mohamed Morsi clashed with opposition activists besieging a mosque in the Mediterranean city of Alexandria.

Friday night’s clashes come just hours before voting is to start around the country on a divisive new constitution.

The protests started after a Muslim cleric appealed for support for the constitution, and protesters surrounded the mosque where the cleric took refuge. 

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Al Jazeera’s Rawya Rageh reporting from near the mosque in Alexandria said the street battles were "raging" well into the evening.

"Ambulances have been rushing in every direction, we saw several people being carried out with bloodied faces," Rageh said, describing those injured in the clashes.

As the standoff continued, more of the cleric's supporters were arriving and clashing with those outside the mosque, at which time the police began fired tear gas at both groups, Rageh said. 

The National Salvation Front of opposition groups has called on its supporters to vote "no" in a constitutional referendum that is scheduled for December 15 and 22.

Rival rallies

Al Jazeera's Rory Challands, reporting from the Egyptian capital Cairo, said the opposition fears the constitution "gives too much emphasis on Islamic law ... They would like to see more emphasis given on rights and freedoms".

Religious authorities had issued orders that mosques should not be used to manipulate Saturday's vote

In particular, those planning to vote against the draft constitution want additional rights for workers and women, Challands said.

The Coalition of Islamist Forces is describing a vote in favour of the draft as "yes" for Islam and has staged its own rally at Nasr City in the capital following mid-day prayers.

The draft constitution, approved by the constituent assembly last month, has become the focus of Egypt's worst political crisis since the June election of President Morsi.

Rival mass rallies held by both supporters and opponents of Morsi have become almost a daily occurrence in Cairo.

Clashes between the two groups killed at least seven people and injured hundreds more last week.

Voting has been taking place at Egyptian embassies abroad and Egyptians abroad are voting for a third day, with more than 500,000 people expected to cast their votes in 150 countries.

Source: Al Jazeera and agencies