Opposition parties in Pakistan have strong leads in elections, a spokesman for a party close to President Pervez Musharraf has conceded.
The PML-Q party would be consigned to the opposition if early results were confirmed, said spokesman Tariq Azeem.
Supporters of opposition parties including Nawaz Sharif's PML-N and the PPP of the late Benazir Bhutto have been celebrating in the streets.
Mr Musharraf insists he will accept the result, regardless of who wins.
He is not standing in the parliamentary election himself, but a clear defeat of his supporters could herald struggles over his presidency, analysts suggest.
The poll took place on Monday, having been delayed following Ms Bhutto's assassination on 27 December.
High-profile victims
Early results showed a "big gain" for Mr Sharif and Ms Bhutto's parties, Mr Azeem told the AFP news agency.
"If the results are confirmed we will play the part of the opposition as effectively as we can," he said.
Most official counts will not be declared until later on Tuesday, and correspondents caution that it is still to early to be sure of the overall trends.
But high-profile victims of the poll were reported to include party president Chaudry Shujaat Hussain and his close ally, Railways Minister Sheikh Rashid.
They were among the PML-Q losses in Punjab, the country's most populous province and a key electoral battleground.
"The result will be the voice of the nation and whosoever wins we should accept it - that includes myself," said Mr Musharraf.
Turnout
Celebrations were continuing in the streets of cities including Lahore, Rapalwindi and Karachi as news of early results trickled out.
At least 20 people are reported to have died in election-day violence, with the PPP reporting 15 of its members killed.
There were also reports of missing ballot boxes.
But there was also widespread relief that there were none of the major bomb attacks which had marred the run-up to the election.
Fears of violence had dissuaded many of the country's 80 million eligible voters from leaving their homes, and voter turnout was estimated to be less than 40%. Close to half a million security personnel, including about 80,000 soldiers, had been deployed to quell any outbreak of fighting.
The wait for news of the next government is the start of fresh uncertainty in Pakistan, says the BBC's Jill McGivering in Karachi.
Even once the election results are clear, there will still be plenty of behind-the-scenes bargaining to come, she says.
Future in the balance
Polls suggest a fair vote is likely to result in a hung parliament, with none of the three biggest parties winning a majority, analysts suggest.
