"The way the people's mandate has blatantly been insulted, it is intolerable", Pakistan Muslim League-Nawaz president and Sharif's brother Shahbaz, told a news conference as the counting continued.
"What they have done has pushed Pakistan back 30 years ..."
"People will not bear it", he added.
The comments came as political parties in Pakistan, including the incumbent Pakistan Muslim League-Nawaz, too raised allegations that the election was manipulated and rigged in PTI chief Khan's favour.
Pakistan is ready to improve its ties with India, Pakistan Tehreek-e-Insaf chief Imran Khan said today, asserting that the blame game between the two neighbours, detrimental to the sub-continent, should stop.
"Our polling agents are being kicked out".
Polling officers count ballots at a polling station in Islamabad.
However, final results have been delayed and opponents have alleged vote rigging.
"All the parties were being cornered just to favor one party - the PTI", she said. He charged fraud and vowed not to accept the results, generating fears that disgruntled losers could delay the formation of the next government.
The Trump administration said it was closely monitoring the situation in Pakistan but refused to declare the polls "free and fair". Violent clashes were also reported in at least four other towns and cities across the country. A charismatic leader, he had a reputation as a playboy and married several times.
Wednesday's voting was marred by a suicide bombing that killed 31 people near a polling station in Quetta, capital of the southwestern province of Baluchistan.
In the run-up to the election, in India, Khan was projected as a Pakistani leader with strong anti-India sentiments with some TV channels dubbing him as the "axis of anti-India" sentiment in Pakistan. In 2004, the couple announced they had divorced amicably, ending the nine-year marriage because it was "difficult for Jemima to adapt to the political life of Imran Khan in Pakistan".
If Khan's lead holds, his party will likely be able to form a government with smaller parties and independents, avoiding the prospect of weeks of haggling.
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.
Confusion prevailed over the Pakistan general elections with the vote counting progressing at a tediously slow pace despite polling ended more than 12 hours ago.
There are 105, 955,407 registered voters in the country who will be voting for candidates for 849 seats in the national and provincial assemblies.
But he was dogged by the accusations he was benefiting from a "silent coup" by the generals which targeted the PML-N. "I promise masses that I will protect the money of taxpayers", articulated Khan. The Islamic State (IS) extremist group claimed responsibility for the attack. "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. Khan's party has ruled the province for the last five years.