Pakistan's Attorney General, Malik Qayyum, has rejected allegations that he said there would be massive rigging in Monday's election.
Mr Qayyum has dismissed Human Rights Watch's claims
Human Rights Watch released an audio recording, in which it says Mr Qayyum makes the comments about rigging.
Mr Qayyum says the allegations are "a conspiracy against Pakistan".
Opposition parties say there have been many irregularities in the run up to the vote, which President Pervez Musharraf has promised will be fair.
The US-based Human Rights Watch says Mr Qayyum made the comments over the phone to an unknown person in November.
A voice can be heard on the recording, speaking Punjabi and Urdu, in which he says "they will massively rig to get their own people to win".
"I have never uttered these words," Mr Qayyum told the AFP news agency.
Mr Qayyum is a close ally of President Musharraf.
The president will not be standing in the parliamentary vote on 18 February.
But the former military leader could face a serious challenge to his authority if the vote produces a hostile parliament packed with his opponents.
The recording released by Human Rights Watch was reportedly made on 21 November last year, just after an announcement confirming the elections would be held on 8 January.
Pakistan's government insists the polls will be free and fair
That date was postponed to 18 February following the assassination in December of the former prime minister and opposition leader, Benazir Bhutto.
A voice on the recording, purportedly Mr Qayyum's, can be heard saying: "If Benazir takes part, she too will be in trouble."
"They will massively rig to get their own people to win," the voice says.
Human Rights Watch says the recording was made by a journalist who had been interviewing Mr Qayyum.
Mr Qayyum had allegedly interrupted the interview to take a call on another phone.
The recording consists of a single male voice, speaking in Punjabi and Urdu, interspersed with pauses.
The Pakistani People's Party (PPP) of assassinated former PM Benazir Bhutto says it will consider street protests if it suspects electoral fraud.
Mr Musharraf has however warned against any such action.
He stepped down as army chief late last year and this election is seen as another key step in Pakistan's transition from military to civilian rule.
Observers and opposition leaders have, however, warned it is unlikely to be free or fair.