Egypt is voting in a second stage of a constitutional referendum that has sparked weeks of unrest.
Opponents of President Mohammed Mursi 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 Mursi's supporters say the constitution will secure democracy.
The BBC reports 250,000 security personnel have been deployed nationwide to try to keep order during the referendum.
Polling stations opened at 8am local time on Saturday. They had been scheduled to close at 7pm but will now remain open until 11pm. Voting was also extended in the first leg.
Ballots are being cast in the 17 provinces which did not vote in the first round on 15 December.
Turnout was reported to be just above 30% in the first round.
Official results are not expected until Monday, after appeals are heard. If the constitution passes, parliamentary elections must take place within three months.
Analysts suggest the document will be passed. People casting votes in favour of the charter said they were voting for stability.
The BBC reports Egypt's latest crisis began on 22 November, when Mr Morsi issued a decree granting himself broad powers.
The decree stripped the judiciary of any power to challenge his decisions.
After an outcry, the president revoked much of the decree, but refused to back down on the draft constitution.
The text was passed by a constituent assembly that was boycotted by liberals 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.
Resignation of VP
vice-president announced his resignation late on Saturday.
Mahmoud Mekki, who was appointed in August, said the role did not suit his background as a judge.
Mr Mekki, 58, said he had tried to resign on 7 November, but circumstances had forced him to remain.
The Israeli conflict in Gaza and President Mursi's decree on 22 November delayed his decision.
The BBC reports seven of Mr Mursi's leading advisers have quit over the past month.