AVA- With nearly half the votes counted from Wednesday's parliamentary election, Mr Khan's PTI party is in the lead.
It is expected to fall short of an overall majority and to seek coalition partners. Officials deny claims of vote rigging made by Mr Khan's rivals.
Campaigning has been marred by violence. On voting day, a bomb killed 31 people at a polling station.
Mr Khan, the charismatic aristocrat who captained Pakistan to a World Cup victory in 1992, has long shed his celebrity playboy image and has recently faced accusations that his election challenge is benefiting from military interference in the nuclear-armed republic.
With votes counted in 49% of stations, Mr Khan's Pakistan Tehreek-i-Insaf (PTI) party was leading in 119 of the 272 National Assembly constituencies being contested, Pakistan's Dawn Newspaper reported, citing election commission figures.
A total of 137 seats is required for a majority.
This election will mark only the second time that a civilian government has handed power to another after serving a full term in Pakistan.
But the party of disgraced former Prime Minister Nawaz Sharif has rejected the results, as have a host of smaller parties, all alleging vote-rigging and manipulation.
"The way the people's mandate has blatantly been insulted, it is intolerable," Shehbaz Sharif, leader of the Pakistan Muslim League-Nawaz (PML-N) and brother of the former prime minister.