U.S. Congress members 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.
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 eight years of Musharraf's rule - including his alliance with the United States that many Pakistanis oppose, reports Farhan Bokhari for CBSNews.com. An overwhelming victory by the opposition would leave him politically vulnerable, even at risk of impeachment.
Pakistan's security services on Sunday renewed warnings of suicide attacks in parts of the country during Monday's parliamentary elections, a day after a devastating suicide bomb attack came as a powerful reminder of the country's bloodiest election campaign ever.], Bokhari reports.
Campaigning by rival candidates formally closed Saturday night, less than two months after Pakistan's former prime minister Benazir Bhutto's assassination, led to a spate of protests and violence focussing around her home province of Sindh in southern Pakistan.
"In this situation of extremism and terrorism, no agitation, anarchy or chaos can be acceptable" said Musharraf on Thursday, speaking at a government gathering in Islamabad.
Polls open at 8 a.m. and close at 5 p.m. First results are expected late Monday but final, official figures are not likely until Wednesday. The country's 81 million eligible voters will also choose legislatures in Pakistan's four provinces.
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.
With surveys pointing to an opposition victory, anti-Musharraf politicians repeated charges Sunday that the government plans to rig the balloting in favor of the ruling party. They 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."
Ahead of the elections, hospitals in large cities were placed on high alert over the possibility of suicide attacks. Staff from ambulance services also had their leaves cancelled. On Friday, Pakistani military troops began being deployed across the country to remain on standby in support of police in the event that opposition parties stage violent protests if there are fresh allegations of rigging, Bokhari reports.
"This election is among the most difficult elections in Pakistan. Musharraf's position could be weakened if the results are rejected by opposition parties" said Hasan Askari Rizvi, a respected Pakistani scholar on security affairs, in an interview with CBS News.
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.
U.S. congresswoman Sheila Jackson Lee, 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.
U.S. Senator Joseph Biden, who will also 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."
A senior leader in the pro-Musharraf party, Mohammed Ali Durrani, brushed aside talk of vote fraud, saying that the opposition was raising such allegation 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 Sunday, police arrested a ruling party supporter and seized 500 ballot papers that he was allegedly carrying in a car in the Bhutto stronghold of Sindh province, according to regional police official Liaqat Ali.
In Lahore, two gunmen opened fire on supporters of Sharif's opposition party, killing one man and wounding four, senior police officer Ahmad Hassan said. The assailants escaped.
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, 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.
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.