Half of Egypt's eligible voters are casting ballots Saturday for a controversial draft constitution that has sparked weeks of violence and protests.
Men and women are voting separately at schools in Cairo and nine other provinces. Egyptian President Mohamed Morsi has already cast his vote.
Long lines outside polling stations prompted election officials to extend the vote by four hours. Polls are scheduled to close at 11 p.m. local time.
On Saturday, an opposition coalition accused the Muslim Brotherhood, which supports the proposed constitution, of rigging the vote. Mr. Morsi is a former member of the Muslim Brotherhood.
The rest of the country will vote next Saturday . The vote has been scattered over two consecutive Saturdays due to a boycott by a majority of the judiciary, which must oversee the vote by law.
It is not clear when final results will be announced.
Members of the liberal, secular and Christian opposition say they fear the constitution will erode civil liberties because it boosts the role of Islamic law and does not mention women's rights.
Officials say they have deployed 120,000 soldiers to protect polling stations.
Clashes broke out Friday among stone-throwing demonstrators protesting the draft constitution near a mosque in the port city of Alexandria, prompting police to fire tear gas. Medical workers say 15 people were injured.
A largely Islamic committee approved the document last month after liberal and Christian members walked out, complaining they were being ignored.