Clashes have broken out in the city of Alexandria between supporters and opponents of President Mohamed Mursi.
Voting in a referendum on a draft constitution for Egypt is due to begin on Saturday.
Opponents say the draft has been crafted poorly and without proper consultation, and is too Islamist.
President Mursi insists the constitution is needed to complete the transition from former president Hosni Mubarak's rule.
The trouble in the port city of Alexandria reportedly began after a cleric at a mosque urged worshippers to vote "Yes".
Brief clashes were also reported in Nagaa Hammadi, 460km south of Cairo.
However, the BBC reports the protests on Friday appeared to be smaller than the mass rallies of recent weeks.
A BBC correspondent in Cairo said the referendum is about the future direction of Egypt: whether it should be an Islamic country or a secular one, a country with its own distinctive Middle Eastern identity or more in tune with Western and European values.
Polls will open at 8am on Saturday in Cairo, Alexandria and eight other provinces. The other half of the country will vote a week later.
The polling is to be spread out because few judges are willing to supervise the vote.