Pakistan Muslim League-Nawaz president Shahbaz Sharif outrightly rejected the election results, alleging "blatant" rigging, soon after trends showed Imran Khan's party was inching towards the majority mark.
Sharif hinted his party would launch protests against the alleged rigging of polls.
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 influential military.
"Some five other parties including PPP have raised the rigging issue in polls. After consulting them, I will announce the future course of action. Pakistan has suffered today," he told journalists at a press conference.
"We will fight this injustice and use all options," he said, adding that there has been a "blatant violation" of the mandate.
According to Geo TV, Khan's Pakistan Tehreek-i-Insaf was leading on 110 seats followed by PML-N on 68.
Bilawal Bhutto's Pakistan Peoples Party was ahead on 37 seats and Independents on 23, trends showed.
"This is a horrible situation which I have never seen in my 30-year political career. These were the dirtiest polls in Pakistan's history. The results of several constituencies have been withheld where PML-N candidates were winning. We will not accept this justice," Sharif said.
PML-N senator Mushahid Hussain Syed too raised similar doubts on the election results.
"This is the first time in Pakistan's history that five political parties have called a foul play in the polls and termed them massively rigged," he said.
A party can form the government if it manages to clinch 172 seats out of the total 342.
A single party will need at least 137 of the 272 directly-elected seats to be able to form the government on its own.
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