Imran Khan says he has been stopped by US immigration officials and questioned about his views on US drone strikes in his country.
He says he was taken off his flight from Toronto to New York on Friday.
As leader of the Pakistan Movement for Justice party (PTI), he has campaigned for an end to drone strikes against suspected militants in Pakistan.
He says the strikes kill many civilians and foster support for militants.
"I was taken off from plane and interrogated by US Immigration in Canada on my views on drones. My stance is known. Drone attacks must stop,"he wrote on Twitter.
He was said to have been questioned for about one hour, and said the delay meant he missed his flight and a party fund raising event in New York, but said "nothing will change my stance".
The US immigration authorities say they cannot divulge any information on particular cases due to privacy laws, according to the Toronto Sun.
"Our dual mission is to facilitate travel in the United States while we secure our borders, our people, and our visitors from those that would do us harm like terrorists and terrorist weapons, criminals, and contraband,"one official said.
Earlier this month, Mr Khan led thousands of supporters on a march to the edge of the restive tribal areas where drone strikes target militants.