"The greatest strength of democracy is a solid opposition, which was not there in Pakistan for the past 10 years. Till now there was friendly opposition. We will tell the people the benefit of a good opposition," Khan, 60, said in a video released from the hospital in Lahore where he is recuperating from a fall that fractured his spine.
Khan, the chief of Pakistan Tehrik-e-Insaf party, did not acknowledge the PML-Ns victory or congratulate Nawaz Sharif as most other Pakistani politicians have done.
The video was Khan's first reaction to trends which showed that the PML-N was set to bag more than 125 of the 272 parliamentary seats that went to the polls yesterday.
PML-N chief Nawaz Sharif has already claimed victory and said his party will form the next government.
Khan often claimed during the campaign that his party would sweep the election but trends showed that it would bag just over 30 seats.
Analysts pointed out that its actual total could be lower as several leaders like Khan had won in more than one seat and would be allowed to retain only one.
Despite the poor showing, Khan alleged that his party had been affected by rigging in Punjab and Sindh provinces.
The Pakistan Tehrik-e-Insaf will produce a "white paper" on the alleged rigging so that such practices could be prevented in future, he said.
He said his party would form the government in northwestern Khyber-Pakhtunkhwa province, where it has emerged the single largest party.
He pledged to make the province an example for the rest of Pakistan by reforming the police force, education and healthcare.
Khan welcomed the high turnout in the election, describing it as a "democratic evolution". He said Pakistan had witnessed possibly the largest turnout in its history.