With 47 percent of the nationwide polling stations counted, Khan's Pakistan Tehreek Insaf (PTI, or Pakistan Movement for Justice) is winning 113 of the 272 seats up for grabs in the lower house of parliament, known as National Assembly.
Apart from these parties, several dozen other parties are also taking part in the elections.
Cricket star turned politician Imran Khan has declared victory for his party in Pakistan's historic elections, promises a "new" Pakistan.
According to surveys held by Pakistani think tanks, a neck-to-neck contest is expected between the PML-N and the PTI.
Throughout the night and into Thursday, Khan supporters celebrated outside party offices countrywide. However, his attempt seems to have gone in vain with Imran Khan's PTI inching close to form the next government in Pakistan.
The National Assembly members are elected through direct franchise for five years, whereas senators are elected by National Assembly members for a period of six years.
The remainder in the 342-seat lower house of parliament included reserved seats for women and minorities.
After the boycott of the 2008 general elections, Khan baffled many rival political parties by igniting thousands of Pakistanis to public rallies in Lahore and Karachi in late 2011.
In the provincial assembly constituencies, the PTI was leading on 60 seats; ANP on 10; MMA on six; PML-N and PPP on two each and independent candidates on four seats.
Addressing a news conference, Sherry Rehman, the PPP leader and opposition leader in the Senate, said: "All the parties were being cornered just to favor one party - the PTI".
But supporters of jailed former Prime Minister Nawaz Sharif said the vote count was rigged and called it an assault on democracy in a country that has a history of military rule.
Nawaz Sharif's two previous governments were dismissed on corruption charges in 1992 and through a bloodless military coup in 1999, respectively.
Sharif was ousted from power a year ago and jailed over a corruption conviction days before the vote, removing Khan's most unsafe rival.
His brother Shahbaz is leading the party's campaign.
His main challenger is Sharif, whose conservative PLM-N party is now run by his brother Shehbaz. It has a solid vote bank in parts of KhyberPakhtunkhwa (KP) and Balochistan provinces, but the party is mainly relying on Punjab.
"I will reject the election results", said Sharif, who has accused the military of backing Khan to deny his party victory.
The uncertainty of the outcome of the vote - no single party appeared assured of a simple majority win - could also lead to prolonged post-election jockeying that would hamper the forming of the next government.
This time it made it publicly known that it favored Khan in general elections, but did not interfere in the elections itself, he said in response to a question.
"I wish someday the two neighbors have an open border", he said.
The PPP has ruled Pakistan more than any other party since 1970.
In a speech peppered with populist pledges, Khan promised to create jobs for the poor and announced he would turn the palatial prime minister's official residence in the capital into an education facility instead of living in it.
Still, it was the allegations against rigging that may impact Mr Khan's ability to form a government.
Talking to APP, former minister Sardar Yaqoob Khan Nasar, contesting from the same constituency, said, "The constituency (earlier known as NA-263) had witnessed only 26.6 per cent women turnout of a total of 93,016 female registered voters in the last general election of 2013 due to less number of dedicated polling stations for women".
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