Pakistan is to hold general elections on 11 May in what will be its first democratic transfer of power.
President Asif Zardari named the date after parliament completed its full five-year term, a first in a country with a history of military coups.
Talks are under way to form a caretaker administration ahead of the vote.
Parties led by ex-PM Nawaz Sharif and cricket star Imran Khan are expected to pose the greatest challenge to the outgoing Pakistan People's Party (PPP).
The PPP won by a landslide in 2008 following the assassination of Mr Zardari's wife, Benazir Bhutto.
The democratic transition is taking place against a backdrop of continuing militancy and growing sectarian unrest, as well as mounting economy problems.
Elections are also to be held for Pakistan's four provincial assemblies.