Operations at Canada's largest airport were disrupted for several hours Thursday morning after airline staff accidentally led arriving passengers through the wrong door.

The mistake resulted in thousands of passengers at Toronto Pearson International Airport to be grounded as transborder flights in Terminal 1 were delayed.

"A ground handler contracted by one of the airlines directed passengers arriving from a flight to the wrong area of the airport," Sean Davidson, a spokesperson for the Greater Toronto Airports Authority (GTAA), told CP24 earlier on Thursday.

"These passengers are now mixed with passengers who are heading to the U.S., so while there is no imminent security threat here at the airport, U.S. customs now needs to do some work to figure out exactly who has been screened and who hasn't and unfortunately that just takes a bit of time."

Images sent to CP24 showed hundreds of passengers waiting in the terminal, along with airline staff, police, and members of U.S. Customs and Border Protection (USCBP).

Passenger Michael Kuluva, who was waiting to board a flight to Miami, told CP24 on Thursday morning that the area had been in lockdown since about 7:30 a.m.

"We don't know a lot about it," Kuluva said.

"Customs is here, the police is here. No one is able to move in or out. No flights are in or out either."

Shortly after 11 a.m., the issue was resolved, and regular operations resumed. The airport said operations in the domestic and international areas of Terminal 1 were unaffected, as well as all zones in Terminal 3.

"Those outbound passengers going to the U.S. have all cleared U.S. Customs and Immigration. So theoretically, those passengers are sitting at Pearson Airport are literally sitting in a U.S. territory," said aviation management expert John Gradek.

"It tells you the level of diligence people working in an airport must have. You have to be on the ball 24/7 to know exactly what you're doing to follow instructions. Otherwise, we saw the consequences of that this morning."

Davidson said passengers impacted by the delay would need to speak to their individual airlines for any possible compensation.

"We empathize with any passengers this morning that were delayed as a result of this issue," he told CP24.

The airline involved in the mix-up was Lynx Air, and in a statement, a spokesperson for the company said the staff member responsible was not an official Lynx employee.

"We work with a separate ground services operator that is accountable for gate services once our aircraft lands," the statement read.

"We are actively in contact with both the GTAA and the ground handling company to investigate what has happened and how this situation could have occurred."