Egyptian groups are holding rival rallies before voting begins on a contentious draft constitution.
Voting is to start on December 15 in Cairo, Alexandria, and eight other provinces, while the other half of the country will vote a week later.
Tens of thousands of police and troops have been deployed ahead of the vote.
Critics say the draft constitution, backed by President Muhammed Morsi and his allies, is too Islamist.
The main opposition group, the National Salvation Front, has urged a "no" vote but stopped short of calling for a boycott.
The controversy has sparked spasms of violence over the past three weeks between pro- and anti-Morsi camps across the country.
At least eight people have died.
Morsi offered a concession to the opposition by annulling a November decree that gave him sweeping new powers.
Based on reporting by AP and Reuters