The passengers were stuck on the tarmac for another two hours before the flight took off for the trip to Montreal, which took about 30 minutes.
Passengers were fuming as they waited in the stifling plane, where food and water became scarce.
"The plane actually lost power and went zero AC, and then now we've got the doors open and one kid is puking, and people are just losing their minds," passenger Laura Mah told Canada's CBC News as she sat on board. "They're just getting mad, saying, 'This is not all right, this is not OK, you can't do this to us.' The police are in here and the fire department's in here, and they're telling us that they can't do anything, that we just have to stay put."
Mah was also one of the passengers tweeting at the airport and the airline during the ordeal, begging for something to be done.
Other passengers tweeted video of the situation, including one video taken when the plane lost power and went completely dark.
The eight-hour flight finally made it to Montreal around 11:30 p.m., more than eight hours behind schedule. The passengers had been on the plane for a total of about 15 hours by that point.
A second Air Transat flight diverted to Ottawa on the way from Rome to Montreal sat on the tarmac for four hours, CBC News reported. Passengers were not allowed to deplane from that aircraft, either.
It was not immediately clear how many of the diverted airplanes belonged to Air Transat.
In a statement released Tuesday, Air Transat officials put the blame on the airport and the "exceptional traffic" caused by the flight diversions.
"As a result, Ottawa airport staff were unable to provide with loading bridges or stairs that would have enabled the passengers on the Brussels flight to disembark, or our ground crews to replenish the aircraft's empty drinking water reservoir," Air Transat officials said in the statement.
Officials for the airline said the congestion also caused a delay in refueling the plane, which led to the air conditioning outage.
Airport officials denied Air Transat's claims, stating that there were a gate and stairs ready and waiting for the passengers if the airline's staff decided to deplane. Buses were also available to shuttle passengers to the terminal, according to the airport officials' statement.
The responsibility for deciding if a flight will deplane rests with the airline, the statement read.
"Our team was on standby shortly after the first diversion landed, but our services were not requested," airport officials said. "We keep a supply of water, food, diapers and other personal hygiene necessities to support passenger needs in irregular operation scenarios, and were prepared to deploy these supplies.
“Although our staff tried several times to contact the aircrew through the handlers to provide further assistance, the air crew was non-communicative and did not take us up on our offers to assist further,” the statement read.
The Canadian Transportation Agency on Wednesday announced that it had launched an investigation into the delays on the international flights.
The incident comes just two weeks after two Air Transat pilots were arrested in Scotland after they were allegedly found to be too drunk to fly from Glasgow to Toronto. The July 17 flight had to be rescheduled for the following day.