Anti-constitution protesters and Islamists threw stones at one another on Friday, prompting riot police to fire tear gas to separate them. There was no immediate word on possible injuries.
Thousands of Egyptians poured onto streets in Alexandria earlier in the day, waving flags and chanting as they marched through the city. Islamists had called for mass rallies before Saturday's second leg of voting on the proposed constitution.
The measure has the support of President Mohamed Morsi's Muslim Brotherhood and Salafist groups. However, members of the liberal, secular and Christian opposition note it will erode civil liberties because it increases the role of Islamist Sharia law and does not include women's rights.
The proposal was approved by an Islamist committee last month as liberal and Christian members walked out, due to their positions being ignored.
Last Friday, a well-known ultraconservative Salifi cleric was trapped inside of an Alexandria mosque for hours while supporters and opponents of the constitution clashed outside. The cleric had angered some worshippers when he voiced support for the referendum.
The remaining provinces vote will on the referendum on Saturday, a week after voters in Cairo, Alexandria and a group of other provinces cast ballots.
Unofficial results from the first day of polling "show almost 57-percent of voters 'approved' the measure." VoA.