Egyptians are voting in a two-day run-off election to choose their first freely elected president.
Mohammed Mursi, the Muslim Brotherhood’s candidate, is up against Ahmed Shafiq, the last prime minister of ousted President Hosni Mubarak.
The ruling military council has promised to hand over power by 1 July.
But correspondents say many Egyptians who took part in last year’s revolution find the choice uninspiring and some are calling for a boycott.
The build-up to the election has been marred by a Supreme Constitutional Court decision to dissolve parliament.
On Thursday, a panel of judges – appointed by Mr Mubarak – ruled that the law governing Egypt’s first democratic elections in more than six decades was unconstitutional because party members were allowed to contest seats reserved for independents.
Mr Mursi’s Freedom and Justice Party won about 100 of its 235 seats in the People’s Assembly by running candidates for individual seats.
If parliament is dissolved swiftly by the ruling Supreme Council of the Armed Forces (Scaf), whoever wins this weekend’s presidential run-off could take office without the oversight of a sitting parliament, and without a permanent constitution to define his powers or duties.
The vote is being held amid rising political tension and constitutional uncertainty.
A 100-member assembly appointed by parliament earlier this week to draft the new constitution may also be dissolved.
Islamist, liberals and scholars denounced the ruling as a "coup", saying they feared the ruling generals would take back legislative power.
"This series of measures shows that the Supreme Council of the Armed Forces, the head of the counter-revolution, is adamant to bring back the old regime and the presidential elections are merely a show," six parties and movements said in a joint statement that also urged Mr Mursi to boycott the run-off.
On Friday, the Muslim Brotherhood vowed to win the presidency despite the signs of opposition within the judiciary, which is overseeing the vote.
"Isolate the representative of the former regime through the ballot box," said a statement referring to Mr Shafiq, who also served as head of the air force and minister of aviation under Mr Mubarak.
The Brotherhood warned that the progress made since the president was forced to step down was being "wiped out and overturned".
Mr Mursi meanwhile sought to reassure the military and its supporters within the electorate that he would work closely with the generals.
"As president, they will be in my heart and will get my attention… they will never do anything to harm the nation," he said.
His opponent meanwhile told a rally that the court rulings were "historic" and that the "era of political score-settling" had ended.
On Friday, Mr Shafiq promised to "address chaos and return stability".
He came second in last month’s first round, in which turnout among the 52 million eligible voters was only 46%.
Official results gave Mr Mursi 24.8% and Mr Shafiq 23.7%.
Polling stations are due to open on Saturday and Sunday at 08:00 (06:00 GMT) and close at 20:00 (18:00 GMT), but voting is likely to be extended on both days.
Final results from the Higher Presidential Election Commission (HPEC) are due by 21 June, but are expected to arrive much earlier.
Partial results from the first round were declared within 24 hours.