Unofficial tallies show Egyptians have voted narrowly in favour of a constitution shaped by Islamists and opposed by liberals in the first round of a two-stage vote.
Liberal opponents of President Mohammed Mursi said the basic law would divide the nation as it did not represent the aspirations of all Egyptians.
"The referendum was 56.5% for the Yes vote," a senior official in the Muslim Brotherhood's Freedom and Justice Party operations room set up to monitor voting said.
The Brotherhood and its party, which propelled Mr Mursi to power in June, had representatives at almost all polling stations across the 10 areas, including Cairo, where this round of voting was held.
The party official, who asked not to be identified, said the tally was based on counts from more than 99% of polling stations in this round.
The second round vote in remaining areas of the country is next Saturday. Official results are not expected until after that stage is complete.
The opposition had initially said it expected the No vote to prevail shortly after polling stations closed.
But as counting proceeded, another opposition official monitoring the vote, Yahya Arafat of the Popular Current movement, had told Reuters the vote was "very close" and could go either way.