Opposition leaders warned Sunday against massive fraud as Pakistanis prepared to choose a new parliament in an election that could determine the political survival of President Pervez Musharraf — America's key ally in the war on terror.

American representatives urged Musharraf to live up to promises of a free and fair vote, despite opinion surveys pointing to an opposition victory in Monday's election.

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Musharraf was re-elected last October to a new five-year term. But the retired general faces growing public anger over his moves last year to declare emergency rule, purge the judiciary and curb independent media.

The election is broadly seen as a referendum on his eight years of rule — including his alliance with the United States that many Pakistanis oppose. An overwhelming victory by the opposition would leave Musharraf politically vulnerable, even at risk of impeachment.

Public opinion surveys have suggested that if the election is fair, the Pakistan People's Party of assassinated former prime minister Benazir Bhutto will finish first, followed by another opposition party led by ex-premier Nawaz Sharif.

The pro-Musharraf party — the Pakistani Muslim League-Q — is trailing a distant third, according to the surveys.

Anti-Musharraf politicians repeated charges Sunday that the government plans to rig the balloting in favor of the ruling party and warned of street protests if the balloting is manipulated.

"This is not going to be a free and fair election," the spokeswoman for Bhutto's party, Sherry Rehman, told reporters Sunday. "We have improvised polling stations coming up in the last few days. We have firing on our rallies."

Sharif, who was ousted by Musharraf in a 1999 military coup, warned that if the results are rigged, the opposition will launch a nationwide protest movement "from which those rigging it will not be able to escape."

For his part, Musharraf has warned he would not tolerate protests by disappointed opposition parties after the election. That could set the stage for a dangerous confrontation in this nuclear-armed nation.

The election was delayed six weeks after Bhutto died in a suicide gun and bombing attack in Rawalpindi on Dec. 27. Since her death, the campaign has been overshadowed by the fear of violence, which tamped down public rallies and took much of the spirit out of the contest.

A series of deadly suicide bombings have left hundreds dead in past weeks, including at least 40 who died Saturday in a suicide car bomb attack against a campaign rally in northwest Pakistan. More than 470,000 police and soldiers have been deployed throughout the country to guard against further attacks.

Rep. Sheila Jackson Lee, D-Texas, one of several American lawmakers who will monitor the voting, said many Pakistanis had expressed concern that the vote would not be fair and urged Pakistani authorities to guarantee a clean election.

"Democratic, safe, secure, transparent elections is what the world is looking for," Lee said after a meeting with officials of Bhutto's party in Islamabad. She warned that the United States would be "very serious in its response" if the election is flawed.

Sen. Joseph Biden, D-Del., who also will monitor the vote, told reporters Sunday in Lahore that Washington should cut military aid to Pakistan if the elections are rigged.

"Without an open election that gains the confidence of the vast majority of the middle class here, there will be great turmoil," Biden said. "I do not buy into the argument that the only person who has the capacity to help in dealing with terrorism is Musharraf."

However, a senior leader in the pro-Musharraf party, Mohammed Ali Durrani, brushed aside talk of vote fraud, saying that the opposition was raising the allegations because it fears defeat in an election that will be monitored by thousands of local and foreign observers and media.

"No one accepts election results after losing," Durrani said Sunday.

Bhutto's husband, Asif Ali Zardari, appealed for national unity as the election approached.

"I think we have reached the breaking point where if we don't band together we will lose this great nation which we call Pakistan today," Zardari said Sunday during a speech in Islamabad.

With international pressure rising, government officials sought to reassure the public that the vote would be fair and peaceful. Information Minister Nisar Memon warned Sunday that if "anyone tries to create a law and order situation, he will be dealt with sternly," Memon told reporters.

Still, many Pakistanis remained suspicious, given the country's long history of rigged elections.

As voting materials were being distributed to polling centers, police on Sunday arrested a ruling party supporter and seized 500 ballot papers that he allegedly was carrying in a car in the Bhutto stronghold of Sindh province, according to regional police official Liaqat Ali.

In Lahore, two gunmen opened fire Sunday on supporters of Sharif's opposition party, killing one man and wounding four, senior police officer Ahmad Hassan said. The assailants escaped.

In Lahore, gunmen opened fire on supporters of Sharif's opposition party in two separate incidents, killing one man and wounding 14 other people, including a candidate in the provincial election, police said. It was not clear who carried out the attacks.

But some analysts fear that recent opinion polls may have raised false expectations among the opposition, which could lead to violence if the vote count does do not match up with the surveys.

One Western diplomat said the pro-Musharraf party still retains the support of many powerful landowning families in Punjab, the most populous province and the key electoral battleground. Poor farmers traditionally follow the advice of their landlords on how to vote — regardless of personal views.

The diplomat, who spoke on condition of anonymity because of the sensitivity of commenting on Pakistani affairs, predicted that the two opposition parties would fall short of enough seats to form a government and that the country could end up with a broad coalition possibly including the ruling party — a move that would block any move to impeach Musharraf.