"We reject this result", Sharif, who leads the incumbent Pakistan Muslim League-Nawaz (PML-N), told a press conference in Lahore.
Some voters in Lahore, capital of Punjab and traditionally a PML-N stronghold, said they were abandoning the party in favour of PTI.
As per unofficial trends, according to Dawn, Imran Khan's PTI is leading in 112 seats in Pakistan, while PML-N is second with 65 seats.
The Elec Commission sends out the official results in a day or so. Khan's position on the US is far more hawkish than former prime minister Nawaz Sharif, noted Moeed Yusuf, from US Institute of Peace.
Both parties said they would announce their course of action on Thursday, while PTI dismissed accusations of malfeasance.
The Pakistan People's Party (PPP) led by Bilawal Bhutto Zardari - son of slain prime minister Benazir Bhutto - was a distant third, leading in 39 constituencies. The requirement was imposed after the 2013 elections, when several areas banned voting by women, mostly in Pakistan's religiously conservative northwest.
The election has been marred by widespread allegations of pre-poll meddling by the powerful military in favor of Khan, which saw the process dubbed Pakistan's "dirtiest election".
Islamabad, July 25: Voting for the Pakistan Elections 2018 took place today between 8 am and 6 pm (8:30 am and 6:30 pm IST).
Uber driver streamed hidden camera videos on Twitch
"The troubling behavior in the videos is not in line with our Community Guidelines ", a company statement said . But the nature of his recordings and the discussions around them that took place on Twitch has drawn criticism.
"So the Election Commission of Pakistan will have to respond - but it is unclear how", former state department official Alyssa Ayres, who is now with the Council on Foreign Relations, said.
The ECP was also accused of conducting rigged elections due to non-provision of Form 45, a sheet containing the final vote tally at each polling station.
Representatives from several parties said that their polling agents were expelled from polling stations during vote count and were denied certified copies of results - breaching election procedures.
Voting day was marred by a suicide blast that killed almost 30 people near a polling centre in Quetta, the capital of Balochistan province.
A spokeswoman for Sharif's party said there were already "serious reservations" about the vote count after reports that soldiers stationed in polling stations had thrown out political parties' monitors during the tabulations.
"This is a deplorable and cowardly attack on a day when voters across Pakistan should be casting their ballots in a peaceful environment, without fear or hindrance", Michael Gahler, the EU's chief observer to Pakistan, said. Sharif's party claims that their cadres were forced to stay at home which disenchanted their supporters to come out and cast their mandate. "On India, even though Khan is likely to take a more balanced approach, he will certainly not be as forward leaning as Sharif", he said. In this scenario, the PTI is likely to form a coalition government.
As per the fresh updates issued by the Election Commission of Pakistan (ECP), the PTI is leading on 84 seats, followed by the PML-N on 58 and the PPP on 29.
In the ongoing elections, as many as 105.96 million people, including 47 million female voters, will be able to use their right to vote, with over 1,2000 candidates vying for 270 national and 570 provincial assembly seats. Pakistan's military deployed 350,000 troops countrywide at polling stations.
The flagship BBC2 news programme opened with a piece documenting Khan's journey from a cricketing hero to a potential prime minister. Others or the non-aligned candidates, including independents, are ahead in 62 constituencies.