- Voting is under way in Egypt's two-day presidential runoff
- About 50 million Egyptians are eligible to vote
- The election pits Mohamed Morsi against former Prime Minister Ahmed Shafik
Cairo (CNN) -- Egyptians started casting their ballots Saturday in a presidential runoff that pits two candidates whose platforms are at the crux of the country's political chaos.
On one side, an Islamist backed by the political arm of the Muslim Brotherhood. On the other, a former air force general who served as the last prime minister in the waning days of Hosni Mubarak's regime.
And sitting in the background, overseeing the election -- and the creation of a new constitution -- are the military rulers who have been in control of the country since Mubarak was ousted from power last year.
The biggest question facing the at least 50 million Egyptians eligible to cast ballots in the runoff is what happens after voters decide between Mohamed Morsi of the Justice and Freedom Party, the Muslim Brotherhood's political arm, and former Prime Minister Ahmed Shafik.
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A protestor stands on a barricade of barbed wire as Egyptian military police stand guard. Egypt's Supreme Constitutional Court ruled that the Islamist-led parliament must be immediately dissolved. 
An Egyptian boy waves his shoes as he joins supporters of the Muslim Brotherhood in a protest in Cairo's Tahrir Square against Mubarak-era prime minister and presidential candidate Ahmed Shafiq after Egypt's top court rejected on Thursday a law barring him from standing in a tense presidential poll runoff. 
Protestors gesture towards military police through a barricade of barbed wire during a protest against presidential candidate Ahmed Shafiq outside the Supreme Constitutional Court on Thursday. 
People walk past graffiti showing faces of ousted Egyptian president Hosni Mubarak, right; Field Marshal Hussein Tantawi, second right; former Secretary General of the Arab League Amr Mussa, second left, and former prime minister and presidential candidate Ahmed Shafiq, left, at Tahrir square. 
A boy peers through barbed wire at Egyptian military police standing guard outside the Constitutional Court in Cairo on Thursday, June 14. 
Presidential candidate Mohamed Morsi of the Muslim Brotherhood, who is set for a runoff election against Ahmed Shafiq, speaks at a press conference in Cairo on Wednesday, June 13. 
Former Prime Minister Ahmed Shafiq, center, is seated before addressing a business conference in Cairo on Wednesday. 
Egyptians read the front page of newspapers for sale outside of Al-Fatah Mosque in Cairo on Friday, May 25. 
Ballots are counted by election officials in Alexandria as the country eagerly awaits the outcome Friday. 
A supporter of presidential candidate Abdelmonen Abol Fotoh voices her opinions at Tahrir Square on Friday. 
Supporters of various candidates debate outside Al-Fatah Mosque in Cairo on Friday. 
Electoral officials monitor voting in Namul, a village north of Cairo, on Thursday, May 24, the second and final day of voting in Egypt's historic presidential election. Egypt is holding its first presidential election since last year's toppling of Hosni Mubarak, part of the wave of Arab Spring uprisings. 
Egyptian women wait in line Thursday to cast their vote outside a polling station in Cairo. If no candidate gets a majority of the vote in the first round of voting, a second round will be held June 16-17. 
An election worker checks the identification of a voter at a polling place Thursday in Namul as Egyptian soldiers stand guard. 
A soldier stands watch in the Egyptian capital on the second day of voting. A pervasive fear exists that the powerful military, which has run the country since Mubarak's fall 16 months ago, could try to hijack the election. 
An Egyptian man waits to cast his ballot Thursday north of Cairo. The vote is considered Egypt's first free and fair presidential election in modern history. 
An Egyptian man drops off his ballot at a polling station Thursday in Cairo. The voting marks the first time Egypt has held a presidential election in which the results aren't known beforehand. 
An Egyptian woman holds up an ink-stained finger after casting her ballot in Cairo on Wednesday, May 23, the first day of voting in the historic election. 
A voter studies her ballot Wednesday in Cairo. Thirteen candidates are competing in the wide-open race, but two withdrew after ballots were printed. 
Egyptian men fill out their ballots Wednesday in Cairo. Results of the first round of voting are not expected before the weekend. 
An Egyptian man casts his ballot at a Cairo polling station. Some Egyptians told CNN that they waited up to four hours Wednesday to vote. 
Egyptian men shield themselves from the hot sun outside a Cairo polling station Wednesday. 
Egyptian men line up to cast their vote Wednesday in Cairo. Some 30,000 volunteers fanned out to ensure voting is fair, said organizers with the April 6 youth movement, which has campaigned for greater democracy in Egypt. 
Egyptian men fill out their ballots at a Cairo polling place. 
Presidential candidate Hamdeen Sabahy, a leftist dark-horse contender, flashes a sign of victory as he waits to vote at a Cairo school. 
Presidential candidate Abdelmonen Abol Fotoh, a moderate Islamist, casts his ballot Wednesday in Cairo. 
An Egyptian Coptic nun drops her ballot at a Cairo polling station Wednesday. 
Egyptian women wait outside a polling station in Cairo. Many Egyptians seem uncertain of their loyalties to any particular candidate. 
Political turmoil in Egypt Shafik and Morsi emerged from the first round of voting in May to advance to the runoff, in which Egyptian voters are to choose the country's first democratically elected president.
The runoff election was prompted after a May election failed to produce a winner with a clear majority. Polling sites will be open Saturday and Sunday from 8 a.m. (2 a.m. ET) until 8 p.m. (2 p.m. ET).
Votes must be counted by Monday, with final results to be announced by Thursday.
The vote comes two days after a high court ruling that dissolved parliament and raised questions about whether the military will relinquish power.
The Supreme Council of the Armed Forces -- the country's military rulers -- formally dissolved parliament Friday, in line with Thursday's ruling from the nation's top court, which declared the legislative body invalid.
The military council claimed full legislative power after the High Constitutional Court ruled that the constitutional articles regulating parliamentary elections were invalid.
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Gen. Hussein Tantawi, the head of supreme council and Egypt's de facto ruler, met Friday in an emergency session with the council to discuss drafting a new constitution. The council is expected to issue its own interim constitutional charter.
While the court's initial ruling prompted protests, Cairo streets remained relatively quiet compared with the popular demonstrations in February 2011 that brought down Mubarak's regime.
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The Supreme Presidential Electoral Commission has approved licenses for 53 organizations to observe the elections, including at least three international groups -- the U.S.-based Carter Center, the South Africa-based Electoral Institute for Sustainable Democracy in Africa and the Arab Network for Monitoring of Elections.
The election of a president without a parliament means that the winner will wield extraordinary power, dealing directly with the military rulers while a new constitution is written and until new parliamentary elections are held.
CNN's Saad Abedine, Ben Wedeman and Ivan Watson contributed to this report.
