Egyptians vote Saturday on a contentious referendum on a new constitution, following a campaign plagued by violence and protests.

Clashes broke out Friday among stone-throwing protesters near a mosque in Alexandria, prompting police to fire tear gas. Medical workers say 15 people were injured.

In Cairo and other cities, thousands of flag-waving activists turned out for rival rallies. Such gatherings generally have been peaceful.

Officials say 120,000 soldiers will be deployed to protect polling stations Saturday when half the country's eligible voters cast their ballots. The rest of the country will vote December 22.

The proposed constitution has the support of President Mohamed Morsi's Muslim Brotherhood and Salafist groups.

Members of the liberal, secular and Christian opposition, however, fear it will erode civil liberties because it boosts the role of Islamic law and does not mention women's rights.

A largely Islamic committee approved the document last month after liberal and Christian members walked out, complaining they were being ignored.

Last week, clashes between supporters and opponents left eight people dead and hundreds injured.

Meanwhile, the United Nations Working Group has urged Egyptian officials to review the document to ensure that in conforms to international laws on equality and women's rights.

In a Friday statement, independent expert Kamala Chandrakirana said “key opportunities so far have been missed.” The group says the final draft of the constitution does not include adequate provisions to protect against gender discrimination.