"We concede and congratulate the people who have won the elections," Mushahid Syed Hussain, general secretary of the Pakistan Muslim League-Q, told CNN. "We will look forward to strengthening the democratic process by remaining in opposition."
Pakistanis celebrated the toppling of Musharraf's party with newspapers proclaiming, "All the King's Men, Gone!" and "Heavyweights knocked out."
Voter turnout for the first parliamentary elections in six years exceeded expectations, with nearly 46 percent of the 81 million eligible voters casting ballots, CNN reported.
Final results aren't expected before Tuesday night but early returns indicated big wins for opposition parties headed two former prime ministers.
Preliminary results, as reported by GEO TV, showed the People's Party, the party of former Prime Minister Benazir Bhutto, who was assassinated Dec. 27, leading with 87 seats in the 342-seat National Assembly while the Pakistan Muslim League-N of former Prime Minister Nawaz Sharif was next at 66 and the Musharraf-aligned PML-Q third with 38.
Not all seats in the National Assembly were contested in Monday's election.
Should the opposition be able to amass a coalition taking in two-thirds of the National Assembly's seats, the new government could take steps to impeach Musharraf.
Voting ended without overt signs of tampering but 23 people died in election-related violence, CNN reported.