There have been clashes in Egypt's second city on the eve of a final vote on an Islamist-influenced constitution that has divided the country.
Supporters of President Mohammed Mursi and his opponents hurled rocks at each other in the city of Alexandria.
Police fired tear gas to separate scores of opponents of the constitution and thousands of Islamists who clashed near a mosque.
Health officials said more than 20 people were injured.
Mr Mursi and his Islamist allies back the draft constitution as a vital step in Egypt's transition to democracy almost two years after the fall of Hosni Mubarak.
The opposition says the draft, drawn up by an Islamist-dominated assembly, is a recipe for deeper divisions and more violence.
Calling for a No vote, it says the document is too Islamist and ignores the rights of women and minorities, including the 10% of Egyptians who are Christian.
The National Salvation Front, the main opposition coalition, said a No vote meant taking a stand against attempts by the Brotherhood to dominate Egypt.
The run-up to the final round of voting on the new constitution has been marked by often violent protests that have cost at least eight lives.
The first round on 15 December produced a Yes vote, at 57%, that is expected to be repeated in the second round.
The second stage tomorrow is expected to produce a similar result.
The constitution must be in place before a parliamentary election can be held.
If it passes, the poll should be held within two months.
Demonstrations erupted when Mr Mursi awarded himself sweeping powers on 22 November and then fast-tracked the constitution through a drafting assembly dominated by his Islamist allies and boycotted by many liberals.
The referendum is being held over two days because many of the judges needed to oversee polling stayed away in protest.
In order to pass, the constitution must be approved by more than 50% of those voting.