Pakistani politician Imran Khan, a vocal critic of US drone strikes, was briefly delayed and questioned by US immigration officials in Toronto before being allowed to board a flight to New York, prompting his party to demand an apology from Washington.
Khan told his followers on Twitter that he was detained and interrogated yesterday about his views on drones.
A State Department official confirmed Khan had been briefly detained, but said the former Pakistani cricket star was later released to go the United States. "The issue was resolved and Mr Khan is welcome in the United States," the official said.
Ali Zaidi, senior vice president of Khan's Pakistan Tehreek-e-Insaf party, demanded an apology from US authorities for their two-hour questioning of Khan and his travelling companions, as well as a thorough investigation.
The State Department gave no details about why Khan was detained. The US Customs and Border Protection agency said it was prohibited from discussing specific cases.
Khan, who led a protest march to northern Pakistan this month to protest US drone strikes, vowed to continue opposing the deadly attacks. "Nothing will change my stance," he said.
"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," Khan tweeted.
Khan's Pakistan Tehreek-e-Insaf party said the politician arrived safely in New York, the first day of the Muslim Eid-al-Adha holiday, after the delay at the Toronto airport and went directly to a fundraising lunch.
Zaidi, of the party, said the incident violated ethical and diplomatic norms and the Pakistani government should complain to the US embassy in Islamabad.
Calling Khan "a celebrated national hero" and a "global icon of colossal stature," Zaidi wrote on the party's website that to "subject him to such clumsy and vicious treatment speaks volumes about the exasperation induced in the American ranks by his heroic and patriotic-minded opposition to the drone program."
In an email, the US Customs and Border Protection agency declined to comment on the case, but said travellers who wanted to enter the United States bore the burden of proof to establish that they were eligible for admission, and that included overcoming "ALL grounds of inadmissibility."
Pakistani authorities earlier this month stopped a protest led by Khan from entering the troubled region of South Waziristan, a tribal area frequently hit by drone strikes.
Khan blames the Pakistan government for allowing the United States to operate in the country, and has said he will order the Pakistani air force to shoot the unmanned planes down if he wins next year's elections in Pakistan.
Earlier this month, Khan led a march to northern Pakistan to protest the drone strikes, which have killed between 2600 and 3400 Pakistanis, according to the independent London-based Bureau of Investigative Journalism.
Some Pakistanis say Khan is fanning anti-American sentiment to bolster his political career and criticize him for refusing to condemn atrocities by the Taliban or Pakistani army.
Others praise him for reaching out to Pakistan's northern tribal areas and say he is standing up for a war-ravaged population ignored by mainstream politicians.
The United States says the strikes have killed top Taliban and al Qaeda commanders and that civilian casualties are minimal. But it has not said how targets are selected or how the military determines whether the dead were fighters or civilians.