FACTBOX - Pakistan's presidential election

(Reuters) - Pakistan holds a presidential election on Saturday to replace Pervez Musharraf, who resigned on August 18 under threat of impeachment by the coalition government led by the party of assassinated former prime minister Benazir Bhutto.

Musharraf won a new five-year term in October in a vote by the outgoing parliament and provincial assemblies. Critics said his election was unconstitutional.

Traditionally, presidents have been figureheads in Pakistan, with prime ministers holding real power. But at various times, including under Musharraf, constitutions have been amended to give the president more powers.

The office retains the powers Musharraf held though all candidates have said they would strip the presidency of those powers, in particular the ability to dismiss parliament.

Following are some facts about the election:

-- Asif Ali Zardari, Bhutto's widower and now leader of her Pakistan People's Party, looks set to win the vote. His main challenger is expected to be Saeeduzzaman Siddiqui, a former judge nominated by former prime minister Nawaz Sharif's party, the Pakistan Muslim League (Nawaz) (PML-N). The other candidate is Mushahid Hussain Sayed, nominated by the Pakistan Muslim League that backed Musharraf and ruled under him.

-- Presidents are elected by an electoral college made up of members of the two houses of parliament -- the 342-seat lower house National Assembly and the 100-member upper house Senate -- and members of assemblies in Pakistan's four provinces -- Punjab, Sindh, North West Frontier and Baluchistan. In all, the assemblies have 1,170 seats, although the number of electoral college votes is 702 because provincial assembly votes are tallied on a proportional basis.

-- The PPP has a total of 375 members of parliament and provincial assemblies compared with 274 for the PML-N.

-- The vote will be held at the same time in all of the assemblies, beginning at 10 a.m. (6:00 a.m. British time) on Saturday.

-- The vote is by secret ballot.

-- The candidate who obtains the largest number of votes is declared elected president for a period of five years.

-- The new president will be Pakistan's 11th since 1956, when the country became a republic, excluding acting presidents.

Source: Election Commission of Pakistan; National Assembly; Reuters

(Writing by Robert Birsel; Editing by Jerry Norton)