Party president Shehbaz Sharif called a press conference at about midnight in Lahore.
"People will not bear it", he added.
During the count, outgoing PML-N Defence Minister Khurram Dastgir Khan tweeted that official result forms were not given at hundreds of polling stations, decrying what he called "blatant, callous, shameless rigging".
"Our polling agents are being kicked out".
Polling officers count ballots at a polling station in Islamabad.
Authorities said on Thursday that the full results would be delayed due to technical failures in an electronic reporting system, and that tallying was now being conducted manually.
Sharif's Pakistan Muslim League-Nawaz or PML-N was ahead in 66 constituencies, and the Pakistan Peoples Party or PPP, led by the son of assassinated two-time prime minister Benazir Bhutto, led in 39 constituencies.
"When police tried to stop him he blew himself up", he told AFP. The U.S. government has struggled to elicit much in terms of cooperation - the onset of a new domestic political crisis in Pakistan makes such cooperation more unlikely.
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"All the youth of Lahore and of this country support Khan, because he will bring jobs for them", said Mr Zahoor Hussain, 32, an engineer. A charismatic leader, he had a reputation as a playboy and married several times.
Pakistani security personnel gather at the site of a suicide attack near a polling station in Quetta on July 25, 2018. "Inexcusable & outrageous", tweeted Bilawal Bhutto Zardari, the chairperson of Pakistan People's Party (PPP) which was third in the National Assembly with a lead in 43 seats. A single party will need at least 137 of the directly elected seats to be able to form the government on its own.
Gallup Pakistan estimated turnout at between 50 to 55 per cent in an electorate of almost 106 million, similar to the previous contest in 2013.
In the lead-up to the elections, the PML-N also extensively complained about pre-poll rigging in favor of the PTI through the initiation of targeted graft cases and threats/pressures on PML-N members to leave the party. More than 11,000 candidates vied for 270 seats in the National Assembly, and 577 seats in four provincial assemblies. The military deployed some 350,000 troops to secure the country's 85,000 polling stations.
Khan, who captained Pakistan to their World Cup cricket victory in 1992, vowed to tackle widespread corruption while building an "Islamic welfare state".
Pakistan's former cricket star Imran Khan has claimed victory in his country's parliamentary elections.
Pakistan just held a national election Wednesday, but it ended in a major controversy.
The military has rejected the accusations. He did not say who he believes could have rigged the polls, but allegations of manipulating the elections have been made against the country's powerful military.
Khan also raised eyebrows in recent weeks by increasingly catering to hardline religious groups, particularly over the inflammatory issue of blasphemy - sparking fears a win for PTI could embolden Islamist extremists. PML-N alleged that the proceedings and the outcome of the case was influenced by the Pakistani military.
Sharif, 68, was removed by presidential decree in 1993, in a military coup in 1999, and past year by the Supreme Court, which ordered his corruption trial.