Police officers were deployed to keep apart thousands of Islamists on one side and hundreds of opponents of a controversial draft constitution. They had begun to hurl rocks at each other as protests near al-Qaid Ibrahim mosque turned violent. It had also been the scene of similar violence last week.

Egypt's Muslim Brotherhood had previously called for the mass gathering in Alexandria rally to protest against last week's siege of the mosque by anti-Islamists. This in turn had been sparked by a cleric's call for people to vote yes in the referendum on the draft constitution.

Watch video 01:28 Riots in Egypt ahead of constitutional referendum

Friday's protests precede a second and final round of voting on Saturday. The first round on December 15 reportedly resulted in a 57 percent majority in favor.

The second stage is expected to produce more support for the draft from among voters in rural regions who are seen as more conservative and likely to back Islamist President Muhammad Morsi.

Unrest began on Novemer 22 when President Morsi awarded himself sweeping powers and was perceived by the liberal opposition to be pushing constitution draft through an assembly dominated by his Islamist allies.

The referendum is being held over two days because many of the judges foreseen to oversee polling have decided to stay away.

In order to pass, the constitution must be approved by more than 50 percent of those voting.

It must be in place before parliamentary elections can be held. If it passes, those should be held within two months.

rg/ipj (Reuters, AFP, AP)