Egypt holds second stage of vote on draft constitution
Egyptians are voting in a second stage of a constitutional referendum that has sparked weeks of unrest.
Opponents of President Mohammed Morsi have held protests against the draft, saying it favours the Islamists now in power and betrays the revolution that overthrew Hosni Mubarak last year.
Mr Morsi's supporters say the constitution will secure democracy.
Some 250,000 security personnel have been deployed nationwide to try to keep order during the referendum.
Separately, Vice-President Mahmoud Mekki has announced his resignation.
Mr Mekki, who took the post in August, said in a statement read on state television: "I realised a while ago that the nature of politics does not suit my professional background as a judge."
Mr Mekki, 58, said he had tried to resign last month but circumstances required him to remain.
Tension over the draft constitution has led to weeks of violence in Egypt.
Turnout was reported to be just above 30% in the first round, with unofficial counts suggesting some 56% of those who cast ballots voted in favour of the draft.
Polling stations opened at 08:00 local time (06:00 GMT) on Saturday. They had been scheduled to close at 19:00 but will now remain open until 23:00. Voting was also extended in the first leg.
Official results are not expected until Monday, after appeals are heard. If the constitution passes, parliamentary elections must take place within three months.
Ballots are being cast in the 17 provinces which did not vote in the first round on 15 December
The areas are seen as more conservative and sympathetic to Mr Morsi's Islamist movement, the Muslim Brotherhood.
Analysts suggest this will favour a "Yes" vote.
"I'm voting 'No' because Egypt can't be ruled by one faction," Karim Nahas, a 35-year-old stock market broker voting early on Saturday in Giza, told Reuters news agency.
Egyptians casting votes in favour of the charter said they were voting for stability.
In the town of Fayoum, "yes" voter Hanaa Zaki told the Associated Press news agency: "I have a son who hasn't got paid for the past six months. We have been in this crisis for so long and we are fed up."
Opponents of the draft say it fails to protect the freedoms and human rights that they sought in the uprising that toppled Mr Mubarak.
Some have also complained about the role given to Islamic clerics and what they say is a lack of a clear commitment to equality between men and women.
The campaign group Human Rights Watch has said the draft provides for basic protections against arbitrary detention and torture, but fails to end military trials of civilians or to protect freedom of expression and religion.
Ahead of the second round, opposition politicians said they thought further violence was likely.
"I see more unrest," said Ahmed Said, head of the liberal Free Egyptians Party and a member of the opposition coalition.
One voter in Ikhsas village, Marianna Abdel-Messieh, agreed. "Whether this constitution passes or not, there will be trouble," she told AP. "God have mercy on us."
Separately, Mr Mekki said he had tried to resign on 7 November but that events had prevented him from doing so.
These included the conflict in Gaza and Mr Morsi's decree on 22 November that granted the president sweeping new powers and led to the current crisis over the draft constitution.
After an outcry, the president revoked much of the decree, but he refused to back down on the draft document.
The text was rushed through by a constituent assembly dominated by Islamists and boycotted by liberal and left-wing members, as it faced a threat of dissolution by the country's top court.
Egypt has seen mass demonstrations on both sides ever since.