Former Prime Minister Nawaz Sharif's Pakistan Muslim League (PML-N) on Monday demanded an official probe into allegations of rigging in last week's elections.
"We demand a judicial commission to probe incidents that took place on July 25," PML-N leader Khawaja Asif told reporters after a meeting of the party's Central Executive Committee in Lahore.
Asif said he would publish a white paper with proofs of alleged fraud committed during the elections, Efe news reported.
Former cricketer Imran Khan's Pakistan Tehreek-e-Insaf (PTI emerged as the biggest party but short of simple majority in the National Assembly elections. It is yet to seek formal support from smaller parties or independents to form the next government.
The PML-N and Pakistan People's Party, headed by Bilawal Bhutto Zardari, were second and third respectively.
The election result was delayed by more than two days and several parties alleged that their representatives were thrown out of the polling stations during the counting of votes.
The European Union election observation mission said the election process saw "a notable lack of equal opportunities".
Several religious parties have also demanded a re-election and threatened to boycott Parliament.
The elections were the second in Pakistan's history in which a government was able to complete its term and make way for another elected government after being ruled by military dictators for around half of its 71-year-long-history.