Egypt's referendum on a controversial draft constitution has begun in embassies abroad, days before the vote is due to take place at home.
But with many judges refusing to oversee the process, it is unclear whether the vote will take place on one day or continue the next week.
The opposition has called for the entire referendum to be scrapped.
The armed forces chief has announced plans for unity talks on Wednesday in an attempt to end the political crisis.
As tensions surrounding the vote grew, thousands of protesters took to the streets of Cairo on Tuesday in rallies both for and against the proposed constitution backed by President Mohammed Morsi.
The streets of Cairo were described as calm on Wednesday and army tanks remained deployed outside the presidential palace.
Meeting 'as Egyptians'
Egypt's opposition National Salvation Front refuses to recognise the document, arguing that it was drafted by an Islamist-dominated assembly and fails to guarantee women's rights.
Judges are required to supervise the referendum, but according to one of the main judges' organisations as many as 90% of them are refusing to take part in the process.
Some state media, citing government sources, reported that the vote would take place on the two following Saturdays.
The main cities of Cairo and Alexandria, along with eight other governorates, would vote on 15 December, and a further 17 would vote the following weekend, a justice ministry source told the BBC.
But the reports of a staggered vote were later denied by other sources. State-owned al-Ahram newspaper quoted the head of Egypt's election commission as saying the referendum would be held on one day.
With only three days before Egyptians are due to take part in such a highly-charged vote, the exact procedure of the referendum is far from clear.
Voting has, however, begun in Egyptian embassies and consulates abroad, the official Mena news agency reports. An estimated 589,000 expatriates have until Saturday to cast their ballots, it says.
After a day of demonstrations involving opposition supporters and pro-Morsi Islamists, armed forces head and Defence Minister Abdel Fattah al-Sissi appealed to the president and all the country's political parties to meet in Cairo on Wednesday.
Egypt's military has tried to keep out of the political crisis but, in a presidential decree on Sunday, was given powers of arrest until the referendum was over.
In a statement on the military spokesman's Facebook page, Gen al-Sissi issued his appeal "for the sake of Egypt". The talks would not be political, he said, but the parties would sit together as Egyptians.
President Morsi was due to attend the meeting, state-owned al-Ahram newspaper reported.
The opposition has said it will probably be represented at some level, the BBC's Cairo correspondent Jon Leyne says.
They had previously insisted they would only talk if the referendum on the new constitution was postponed, he adds.