Pakistan's presidential election on September 6 is not a popular vote but a ballot of the country's two houses of parliament and four provincial assemblies.
Three candidates are contesting the election, which was prompted by the resignation of Pervez Musharraf in the face of impeachment charges last month.
The frontrunner is Asif Ali Zardari, the widower of former prime minister Benazir Bhutto, who represents the Pakistan People's Party (PPP).
He is being challenged by retired chief justice Saeed-uz-Zaman Siddiqui, backed by former prime minister Nawaz Sharif, and Mushahid Hussain, a close aide of Musharraf.
Here are some key facts about the election.
-- The president will be voted in by an electoral college comprising the 342-seat National Assembly and the 100-seat Senate, as well as members of the country's four provincial assemblies in Punjab, Sindh, North West Frontier Province and Baluchistan.
-- Voting is by secret ballot. It starts at 10:00 am (0400 GMT) and closes at 3:00 pm.
--- Musharraf resigned August 18 over threats of his impeachment and Sharif withdrew backing from Pakistan's coalition government one week later over a dispute with Zardari on the reinstatement of judges sacked by the former president.
-- Zardari is expected to win easily because the PPP and its allies have an overall majority in the assemblies.
-- Zardari became co-chairman of the PPP after the assassination of Bhutto in a suicide attack in December 2007.
Here are some key facts about the powers of the president.
-- Controversial changes by former military ruler Musharraf made to the constitution empower the president to sack the prime minister, his government and to dissolve the elected parliament.
-- The president is the supreme commander of the armed forces and also has the power to appoint chiefs of the three services, provincial governors, the chairman of the election commission and the attorney general.
-- The president also enjoys the power to waive capital punishment and pardon any condemned prisoner.