The Pakistan People's Party of slain opposition leader Benazir Bhutto on Thursday demanded the formation of a judicial commission to investigate complaints of vote-rigging.
The commission should examine allegations of electoral fraud and rigging in Karachi city and elsewhere in southern Sindh province, senior PPP official Lateef Khosa told a news conference.
"Rigging was committed in targeted constituencies" by candidates backing President Pervez Musharraf, he said.
Khosa's allegations followed a Wednesday night news conference in which Asif Ali Zardari, Bhutto's husband, alleged Pakistani authorities were withholding the results of Monday's parliamentary polls and attempting to change them.
Khosa showed reporters ballot papers he said were already stamped in favour of Muttahida Qaumi Movement (MQM) candidates in Karachi. MQM belonged to a coalition that backed Musharraf in the last parliament.
Khosa said his workers seized thousands of such ballot papers.
Khosa also said voting was tainted in Sindh's Shikarpur district where an important PPP candidate, Aftab Shabaan Mirani, was defeated.
Mirani, a former defence minister who joined Khosa at the news conference, said pro-government activists abducted an officer and seized control of polling stations he headed, in order to put fake votes in the ballot boxes.
"We are not asking for the reversal of the whole exercise. We want re-election in some selective constituencies in Sindh province," he said.
PPP and the party of former prime minister Nawaz Sharif handed Musharraf's allies a crushing defeat in Monday's vote. He faces possible impeachment if an opposition government has a two-thirds majority.
The election commission has not yet released the official results, saying that about half a dozen seats have yet to be confirmed.
With votes counted in 258 out of 272 constituencies, the PPP and Sharif's party had a combined total of 153 seats, the election commission said.
The former ruling Pakistan Muslim League-Q (PML-Q) and its allies had 58.