Imran Khan, the Pakistani cricketer turned politician, has been stopped by US immigration officials and questioned about his views on US drone strikes in his country, party officials have said.
Khan, the leader of the Pakistan Movement for Justice party (PTI), has campaigned vociferously for an end to the controversial US campaign of missile strikes against suspected Taliban and al-Qaeda fighters in Pakistan's tribal areas.
The 59-year-old, who was headed to New York, said he was stopped by US officials in Toronto on Friday.
"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.
Khan said the delay meant he missed his flight and a party fundraising lunch in New York, but insisted "nothing will change my stance".
"My stand on drones is very clear. I did not say sorry to them," Khan told Pakistan's GEO news channel.
Ali Zaidi, a senior party leader, demanded "a prompt and thorough inquiry into this sordid episode" and sought "an unconditional apology from the US government".
Earlier this month, Khan, who argues drone strikes are illegal and counterproductive, led thousands of supporters - and a group of US peace activists - on a march to the edge of the restive tribal districts to protest against drones.
Khan says the drone strikes have killed many innocent civilians and are spreading hatred against the US in the troubled region.
The two-day protest march to Tank, the last town before the semi-autonomous tribal belt, included 15,000 of Khan's supporters and dozens of Western peace activists,
It was an unprecedented gesture from a mainstream politician in one of the most dangerous parts of the country, where Taliban and al-Qaeda fighters operate.
Authorities did not allow the protesters to enter the tribal district of South Waziristan - where missiles fired by US drones routinely target fighters - for security reasons and blocked the road to Tank with shipping containers.
Armed groups have killed thousands of people in Pakistan since 2007, and US officials say the drone strikes are a key weapon in the conflict.
But peace campaigners condemn them as a breach of international law, Pakistanis as a violation of sovereignty that breeds extremism, and politicians, including Khan, as a sign of a government complicit in killing its own people.
Khan, who is campaigning ahead of general elections next year, has made opposition to the drone programme a key plank of his party's policy.
Critics accuse him of merely trying to further his own career and of ignoring both atrocities blamed on fighters and alleged abuses by the Pakistani army.
Although leaked US cables have revealed tacit support for the drone strikes from Pakistan's military and civilian leaders, Islamabad has increasingly condemned the programme as relations with Washington have deteriorated.