Egypt's opposition has called for an investigation into allegations of vote fraud in the referendum on an Islamist-backed constitution.
The Muslim Brotherhood, the main group backing the charter, claimed it passed with a 64% "yes" vote.
Official results have not been released yet and are expected on Monday.
If the unofficial numbers are confirmed, it will be a victory for Islamist President Mohammed Morsi.
The allegations look likely to prolong the struggle that has exploded in deadly street violence at times over the past month.
"The referendum is not the end game. It is only a battle in this long struggle for the future of Egypt," said the National Salvation Front, the main opposition group.
"We will not allow a change to the identity of Egypt or the return of the age of tyranny."
The opposition claims the new constitution seeks to enshrine Islamic rule in Egypt and accuses the Islamists of trying to monopolise power.
Critics say it does not sufficiently protect the rights of women and minority groups and empowers Muslim clerics by giving them a say over legislation. Some articles were also seen as tailored to get rid of Islamists' enemies and undermine the freedom of labour unions.
The opposition front said it filed complaints to the country's top prosecutor and the election commission asking for an investigation.