Egyptians are casting ballots in the first presidential elections since veteran ruler Hosni Mubarak was ousted from power in a popular uprising last February.
Voting is taking place May 23 and May 24 at some 13,000 polling stations across the country.
Thirteen candidates are on the ballot, although one candidate has already withdrawn.
The candidates have a range of views, with some having served as officials during Mubarak's nearly 30 years in power. Other candidates represent parties backed by Islamic groups.
The former Arab League head and foreign minister under Mubarak, Amr Moussa, and Muhammad Mursi, the U.S.-educated candidate of the Muslim Brotherhood, are seen as two of the leading candidates.
With no candidate expected to get more than half the votes for an outright first-round victory, a runoff is likely in June.
Some 50 million people are eligible to cast ballots.
Queues were reported outside polling stations in Cairo shortly after voting began on the morning of May 23.
The election will be closely watched to see whether Islamist parties add to their gains after sweeping most of the seats in the first post-Mubarak parliamentary elections that ended in January.
Presidential Powers Uncertain
Mursi, who heads the Muslim Brotherhood's Freedom and Justice Party, was forced to join the race at the last moment after election authorities disqualified the Muslim Brotherhood's first proposed candidate.
In addition to Mursi and Moussa, reports say other likely frontrunners include Abdel Moneim Abol Fotouh and Ahmed Shafiq.
Fotouh is an independent Islamist candidate who has also drawn support from some liberals and Christians.
Shafiq was Mubarak's last prime minister who, like Mubarak, once commanded Egypt's air force.
Until a new constitution is approved, it is unclear what powers the future president will have.
Analysts say the military council of generals who have been running the government since Mubarak was ousted will likely seek to retain some powers in any new constitutional arrangement.
The continued rule by the generals since Mubarak's fall has triggered numerous clashes involving protesters and security forces in Cairo and elsewhere, leading to scores of deaths and injuries.
Based on reporting by Reuters, AFP, and dpa