Egyptians are due to go to the vote in a referendum on a draft constitution that has divided the country and sparked deadly unrest.
President Mohammed Morsi and his Muslim Brotherhood have campaigned heavily in favour of the constitution.
Opponents say it is poorly drafted and too Islamist.
Some 250,000 security personnel have been deployed for the vote. Polls open at 06:00 GMT for the first part of the two-stage referendum.
Cairo, Alexandria and eight other provinces vote on Saturday. The other half of the country will vote a week later.