The front-runner is ex-prime minister Nawaz Sharif, head of the centre-right Pakistan Muslim League-N (PML-N).
But much of the campaign has been electrified by cricket star Imran Khan with promises of reform and an end to corruption.
The charismatic 60-year-old leader of Pakistan Tehreek-e-Insaf (PTI) tapped into a last-minute surge of support after fracturing his spine when he fell from a stage at a campaign rally on Tuesday.
Although he is expected to make a full recovery, he is flat on his back in hospital and aides say he cannot even vote on Saturday.
The outgoing centre-left Pakistan People's Party (PPP) has run a lacklustre and rudderless campaign, with its chairman, Bilawal Bhutto Zardari, too young to run and largely hidden from public view due to Taliban threats.
- AAP