ISLAMABAD, Pakistan (CNN) -- One day after a pivotal vote that will determine Pakistani President Pervez Musharraf's political viability, early returns on Tuesday showed gains for the opposition parties of former Prime Ministers Nawaz Sharif and the slain Benazir Bhutto.
Pakistani election workers count the votes at a polling station outside Peshawar Monday.
Sharif's Pakistan Muslim League-N secured 21 of the National Assembly's 268 seats, while Bhutto's Pakistan People's Party (PPP) gained 18.
By contrast, Musharraf's Pakistan Muslim League-Q won only five seats. But more votes remained to be tallied; as of 7 a.m. (0200 GMT), only 68 races had been decided.
"As far as we are concerned, we will be willing to sit on opposition benches if final results prove that we have lost. This is the trend," The Associated Press quoted Musharraf's Pakistan Muslim League-Q party spokesman Tariq Azeem as saying.
Musharraf said on Monday, "Whatever the result, we will accept it with grace."
The parliamentary elections were Pakistan's first in six years. Final official results were not expected before Wednesday, AP reported.
In the national races, PPP was running strong in its traditional provincial strongholds of Sindh and Baluchistan, Pakistani television cited incomplete returns in reporting.
And Sharif's Pakistan Muslim League-N was reported leading in the cities of Rawalpindi and Lahore.
Sharif was overthrown in the 1999 bloodless coup that brought Musharraf to power; like Bhutto, he returned from exile late last year.
In addition to Bhutto and Sharif's parties, another 47 also fielded candidates in the race.
Balloting for the provincial assemblies showed similar trends as on the national level. The exception was the southwestern Baluchistan province, where 19 of the 51 posts had been decided, and Pakistan Muslim League-Q led with seven of those seats.
If either of two main opposition parties win a two-thirds majority in parliament, they could take steps to impeach Musharraf. That also could happen if the opposition parties together capture two-thirds of the seats in parliament and then form a coalition.
"Of course, it's a referendum for or against him," retired Lt. Gen. Talat Masood, a political analyst, told CNN. "If the opposition parties win, that means it shows it's against him. I think there is a very high stake for him."
Thousands of gun-toting security forces stood watch outside the nation's 64,100 precincts Monday.
Election-related attacks over the weekend left about 40 people dead.
Chief Election Commissioner Qazi Muhammad Farooq said late Monday it was "too early" to comment on turnout.
However, he added, "What is noteworthy is that there have been no incidents inside the polling places. All the incidents have taken place outside polling stations."
Reports from the non profit Free and Fair Elections Network's 20,000 election observers indicated voter turnout was about 35 percent, AP cited Sarwar Bari of the group as saying. Such a turnout would match the 1997 election for being the lowest in Pakistan's history, AP reported.
Security forces reported at least four bombings at or near polling places around the country, and one person was killed, as supporters of Sharif's party exchanged gunfire with members of the ruling Pakistan Muslim League-Q in the northwestern Punjab province. Another exchange between the factions left two people injured in Sindh.
A remote-controlled bomb exploded in a school used as polling station in Swat, a volatile region rife with Islamic extremists, officials said. No one was inside the building at the time. As a police van rushed to the scene, another remotely-detonated roadside bomb went off -- barely missing the officers, officials said.
And police in Sindh arrested 18 people with weapons near polling places, officials said.
"People here are reluctant to come out because, naturally, they worry about what's going to happen," Faisal Kapadia, a commodities trader in Karachi, told CNN on Monday. "If I go out, I try to avoid public places. And yes, a polling station is one of the most public of places. "
The elections had been scheduled for early January but were postponed after Bhutto's assassination in December. About 81 million people were eligible to vote in the elections, but it was unclear how many stayed home fearing violence after a series of deadly attacks have killed more than 400 people since December.
There had been no overt evidence of vote-rigging, but one voter, Habib Channa, told CNN he went to a polling place in the southern city of Karachi to find that his name had "vanished" from the voter rolls.
Channa said he has been voting for 22 years without problems. Yet he said an elections official told him Monday that his name was not on a list of registered voters -- and neither were the names of 11 relatives.
"This is a sign of injustice," Channa said. "It's all being done by the government of Pakistan."
Elsewhere, police detained a man after someone stole about 200 ballots from a polling place. And authorities brought extra ballots to another polling place after a first batch went missing, said commissioner Qamar-Uz-Zaman, the election commissioner in Sindh.
The elections are crucial for the United States. The Bush administration's priority for Pakistan is to deprive al Qaeda of its sanctuary along the country's rugged border with Afghanistan and to reverse the momentum achieved by the Taliban in its attacks on both sides of the border. E-mail to a friend
CNN's Saeed Ahmed in Atlanta and Ingrid Formanek contributed to this report
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