CAIRO (Reuters) - Egypt's vice-president resigned on Saturday as Egyptians voted in a referendum that is expected to approve a new constitution that lays the foundations for the country's transition to democracy but will strip him of his role.
Authorities extended voting by four hours in the second and decisive round of the plebiscite on an Islamist-drafted constitution that the opposition has criticised as divisive and likely to cause more unrest.
Just hours before polls closed, Vice President Mahmoud Mekky announced his resignation, saying he wanted to quit last month but stayed on to help President Mohamed Mursi tackle a crisis that blew up when the Islamist leader assumed wide powers.
Mekky, a prominent judge who said he was uncomfortable in politics, disclosed earlier he had not been informed of Mursi's power grab. However, the timing of Mekky's move appeared linked to the fact there is no vice-presidential post under the draft constitution.
In a resignation letter, Mekky said that although he had held on in the post he had "realised for some time that the nature of political work did not suit my professional background as a judge".
Islamist supporters of Mursi say the charter is vital to move towards democracy, nearly two years after an Arab Spring revolt overthrew authoritarian ruler Hosni Mubarak. It will help restore stability needed to fix a struggling economy, they say.
But the opposition says the document is divisive and has accused Mursi of pushing through a text that favours his Islamist allies while ignoring the rights of Christians, who make up about 10 percent of the population, as well as women.
"I'm voting 'no' because Egypt can't be ruled by one faction," said Karim Nahas, 35, a stockbroker, heading to a polling station in Giza, a province included in this round of voting which covers parts of greater Cairo.
At another polling station, some voters said they were more interested in ending Egypt's long period of political instability than in the Islamist aspects of the charter.
"We have to extend our hands to Mursi to help fix the country," said Hisham Kamal, an accountant.
Queues formed at some polling stations around the country and voting was extended by four hours to 11 p.m. (2100 GMT).
Unofficial tallies are likely to emerge within hours of the close, but the referendum committee may not declare an official result for the two rounds until Monday, after hearing appeals.
As polling opened on Saturday, a coalition of Egyptian rights groups reported a number of alleged irregularities.
They said some polling stations had opened late, that Islamists urging a "yes" vote had illegally campaigned at some stations, and complained of irregularities in voter registration irregularities, including the listing of one dead person.
Last week's first round of voting gave a 57 percent vote in favour of the constitution, according to unofficial figures.
Analysts expect another "yes" on Saturday because the vote covers rural and other areas seen as having more Islamist sympathisers. Islamists may also be able to count on many Egyptians who are simply exhausted by two years of upheaval.
Among the provisions of the new basic law are a limit of two four-year presidential terms. It says the principles of sharia law remain the main source of legislation but adds an article to explain this further. It also says Islamic authorities will be consulted on sharia - a source of concern to Christians and other non-Muslims.
If the constitution is passed, a parliamentary election will be held in about two months. If not, an assembly will have to be set up to draft a new one.
After the first round of voting, the opposition said alleged abuses meant the first stage of the referendum should be re-run.
But the committee overseeing the two-stage vote said its investigations showed no major irregularities in voting on December 15, which covered about half of Egypt's 51 million voters.
Even if the charter is approved, the opposition say it is a recipe for trouble since it has not received sufficiently broad backing from the population. They say the result may go in Mursi's favour but it will not be a fair vote.
"I see more unrest," said Ahmed Said, head of the liberal Free Egyptians Party and a member of the National Salvation Front, an opposition coalition formed after Mursi expanded his powers on November 22 and then pushed the constitution to a vote.
Protesters accused the president of acting like a pharaoh, and he was forced to issue a second decree two weeks ago that amended a provision putting his decisions above legal challenge.
Said cited "serious violations" on the first day of voting, and said anger against Mursi and his Islamist allies was growing. "People are not going to accept the way they are dealing with the situation."
At least eight people were killed in protests outside the presidential palace in Cairo this month. Islamists and rivals clashed on Friday in the second biggest city of Alexandria, hurling stones at each other. Two buses were torched.
The head of the Muslim Brotherhood, an Islamist group that represents Mursi's power base, said the vote was an opportunity for Egypt to move on.
"After the constitution is settled by the people, the wheels in all areas will turn, even if there are differences here and there," the Brotherhood's supreme guide, Mohamed Badie, said as he went to vote in Beni Suef, south of Cairo.
"After choosing a constitution, all Egyptians will be moving in the same direction," he said.
The vote was staggered after many judges refused to supervise the ballot, meaning there were not enough to hold the referendum on a single day nationwide.
The first round was won by a slim enough margin to buttress opposition arguments that the text was divisive. Opponents who include liberals, leftists, Christians and more moderate-minded Muslims accuse Islamists of using religion to sway voters.
Islamists, who have won successive ballots since Mubarak's overthrow, albeit by narrowing margins, dismiss charges that they are exploiting religion and say the document reflects the will of a majority in the country where most people are Muslim.
Additional reporting by Tamim Elyan; Writing by Edmund Blair and Giles Elgood; Editing by Mark Trevelyan and Jason Webb