People in Egypt are voting on a new constitution which has divided the country and sparked unrest.
President Mohammed Morsi and the Muslim Brotherhood have campaigned heavily in favour of the draft document, which may define Egypt for years to come.
Opponents say it is poorly drafted and overly favours Islamists.
The National Salvation Front coalition has accused the Muslim Brotherhood of trying to rig the vote.
However, the ballot, which is staggered over Saturday and a second day of voting in a week's time, appears to be going smoothly with indications of a high turnout.
BBC correspondents at polling stations report a relaxed mood. Many people said they were voting for the restoration of stability in Egypt.
The votingf on Saturday is in Cairo, Alexandria and eight other provinces, a week before the rest of the country.
Some 250,000 security personnel are deployed to safeguard the referendum. More than 51 million people are registered to vote.