Opposition leaders Nawaz Sharif and Asif Ali Zardari have promised street protests if they are robbed of victory.
Heightening fraud fears is the release of a tape in which Malik Qayyum, the Pakistani attorney-general, is purportedly heard saying that Monday's elections will be rigged.
The attorney-general denied it was his voice on the tape but Human Rights Watch, the group which released the tape, insists it was him.
However, Kanwar Dilshad, the number two figure in Pakistan's election commission, has said there was "no chance of rigging" and that a level playing field has been provided to all parties.
"We are neutral. A level playing field has been provided to all the contesting candidates, and we are doing our job to ensure free, fair, transparent and peaceful elections."
Elections will be held for a new parliament and provincial assemblies.
The elections are crucial for Pervez Musharraf, the Pakistani president. If a parliament hostile to him is elected, it could limit his power or he could be impeached.
If that happens, the US with whom Musharraf is strongly allied in their war on terror, could pull their military and financial support from the country.
The electoral commission has been criticised by human rights groups, who say that it has dismissed complaints of harassment against candidates opposed to Musharraf's government.
Brad Adams, the Asia director of Human Rights Watch, said: "There have been numerous complaints of improper government assistance to the ruling party and illegal interference with opposition activities.
"But the election commission has done nothing significant to address these problems, raising serious questions about its impartiality."
Saturday is the last day for campaigning.
Pakistan People's Party: Benazir Bhutto's former party. Now led by her widower Asif Ali Zardari and son Bilawal Bhutto Zardari
Pakistan Muslim League-N: Led by two-time prime minister Nawaz Sharif, ousted by President Pervez Musharraf in 1999
Pakistan Muslim League-Q: incumbent centre-conservative government. Supports Musharraf
Muttahida Qaumi Movement (MQM): Supports Musharraf. Expected to gain a low number of seats
It follows months of violence including the assassination of Benazir Bhutto, the former prime minster and leader of the Pakistan People's Party (PPP).
The insecurity, suspected corruption and falling living standards in the country have led to disaffection with politics.
Voter turnout is expected to be low, analysts say, with a local Gallup poll showing that only 51 per cent of people believed that the elections would be free and fair.
More than 470,000 soldiers have been assigned to keep the peace at the elections.
The three main parties in the fray are Bhutto's PPP, the Pakistan Muslim League(N) of Sharif, another ex-prime minister, and the Pakistan Muslim League(Q), allied to Musharraf.