Egyptians are awaiting the results of the second phase of a referendum on a divisive constitution.
Both sides say early results indicate it has passed, but the opposition has complained of voting irregularities. Final results are expected on Monday.
Critics say the draft document favours Islamists and betrays the revolution that overthrew Hosni Mubarak last year.
As voting took place on Saturday, the country's Vice-President Mahmoud Mekki announced his resignation.
Over the past month, seven of President Mohammed Morsi's 17 top advisers have resigned.
Mr Mekki, a former judge who was appointed vice-president in August, said the "nature of politics" did not suit his professional background.
Late on Saturday, state television announced that the central bank governor, Farouq al-Uqdah, had also resigned from his post. However, a cabinet official later denied the report.
Mr Morsi's supporters say the proposed constitution will secure democracy and provide stability for the weak economy.
But its opponents accuse the president of pushing through a text that favours Islamists and ignores the rights of Christians, who make up about 10% of the population, and women.
On Saturday, ballots were being cast in the 17 provinces that did not vote in the first round on 15 December.
The Islamist Muslim Brotherhood movement said early on Sunday that with most votes counted, 71% were in favour.
The opposition National Salvation Front also said the "yes" vote appeared to have won.
An official from the Front told Reuters that the Islamists were "ruling the country, running the vote and influencing the people, so what else could we expect?"
One Egyptian, 19-year-old law student Ahmed Mohammed, said he voted "yes" because Egypt "needs a constitution to be stable".
But at the same polling station in Giza, south-west of the capital, 50-year-old housewife, Zarifa Abdul Aziz, said: "I will vote 'no' a thousand times. I am not comfortable with the Brotherhood and all that it is doing".
In the first round, on 15 December, turnout was reported to be just above 30% with unofficial counts suggesting some 56% of those who cast ballots voted in favour of the draft.
Official results are not expected until Monday, after appeals are heard. If the constitution passes, parliamentary elections must take place within three months.
Mr Mekki, 58, announced his resignation just hours before the end of voting on Saturday.
He said, in a statement read on television: "I realised a while ago that the nature of politics does not suit my professional background as a judge."
Mr Mekki said he had tried to resign on 7 November but that circumstances had forced him to remain.
The Israeli conflict in Gaza and President Morsi's controversial decree on 22 November granting himself sweeping new powers delayed his decision.
Mr Mekki's resignation statement indicated he had no prior knowledge of the decree, which stripped the judiciary of powers to question the president's decisions.
After an outcry, the president revoked much of the 22 November decree, but he refused to back down on the draft constitution.
The text was rushed through by a constituent assembly dominated by Islamists and boycotted by liberal and left-wing members, and facing a threat of dissolution by the country's top court.
Egypt has seen mass demonstrations by both sides, which have occasionally turned violent, ever since.