US Senator Joseph Biden says Washington should cut military aid to Pakistan if Monday's parliamentary elections are not credible and fair.
Biden, who is in Pakistan to observe the polls along with several US lawmakers, urged Pakistani authorities to ensure that the vote is free and fair. He warned the country could face greater instability if the vote is rigged.
Security forces throughout Pakistan are on high alert in the hours leading up to the balloting.
Nearly 400 thousand police and 80 thousand troops are providing security for the vote, which follows months of politically motivated violence that has killed hundreds of people.
The elections are considered crucial to restoring democracy in Pakistan, following eight years of military rule under President Pervez Musharraf.
Early results are expected late Monday, but final official figures may not be available for several days.
Monday's vote will determine the makeup of Pakistan's National Assembly and provincial assemblies. If opposition groups win a large majority in parliament Monday, they could try to impeach Mr. Musharraf, whose popularity has plunged steeply during the past year.
The opposition, however, says that a massive vote-rigging operation is in place, making a free and fair election impossible. They have vowed to launch a nationwide protest movement if the election is fraudulent.
Information Minister Nisar Memon rejected the charges, saying that vote counting will be transparent. The government says it has issued entry visas for more than one-thousand election observers and foreign journalists.
In other news Sunday, police say at least four Pakistani soldiers died when a military vehicle in Baluchistan province hit a land mine near the town of Dera Bugti. A rebel group says it carried out the attack, in which at least eight troops were killed.
Another person was killed and at least five others wounded in Lahore when a gunman opened fire on the office of an opposition candidate.