Passengers on an Air Canada flight say they were left praying in their seats after their plane suddenly lurched in the sky and one of the pilots was pulled from the cockpit during an apparent medical emergency.

The terrifying ordeal happened Wednesday on Air Canada Flight AC7664, which was being operated by regional partner PAL Airlines. The flight was traveling from Newark, New Jersey, to Halifax, Nova Scotia, when the captain suffered a medical emergency midair, the airline said.

The first officer quickly took control of the De Havilland Q400 and diverted the aircraft to Boston, where it landed safely.

But inside the cabin, passengers said they knew almost immediately that something had gone terribly wrong.

“The moment the plane swerved, I knew something was wrong because it was not turbulence,” passenger Rodney McDonald told ABC News. “It really felt like someone had jilted the controls and then it happened over and over again.”

McDonald was traveling with his wife and two sons when the flight suddenly became a nightmare.

“Every thought goes through your mind,” he said. “You start praying. My boys instantly started praying.”

Moments later, he said, a flight attendant rushed into the cockpit in a panic. A short time after that, the crew member dragged one of the pilots out into the aisle.

“One of the flight attendants entered the cockpit frantically and a few moments later, he was dragging one of the pilots out of the cockpit onto the aisle way of the airplane,” McDonald recalled.

McDonald said the pilot appeared to be having a seizure. He and about four other passengers jumped in to help as the first officer continued flying the plane toward Boston.

“It was really horrifying,” McDonald said.

He said he had been sitting in the front row while his family was seated farther back. When he went to be with them, he realized the pilot was physically out of control, though not violent.

“It was clear that he was not in control of his faculties and needed to be restrained,” McDonald said.

For roughly 40 minutes, McDonald said, passengers worked to keep the pilot secure.

“We worked to get him under control,” he said. “It was a fairly strenuous 40 minutes of keeping him down and using as many seatbelts as we could to restrain his legs, arms and chest.”

The Centers for Disease Control and Prevention notes that people who may be experiencing a seizure should not be held down because of the risk of injury. Instead, the agency advises clearing the area of dangerous objects, gently turning the person on their side and keeping their airway clear.

A registered nurse who happened to be on board helped direct passengers and assist the pilot during the emergency, McDonald said.

He also praised the flight attendants for staying composed while the cabin descended into fear and confusion.

“The flight attendants were stupendous,” he said. “They stayed calm.”

There were 61 passengers on board the flight, according to Air Canada. Emergency responders met the aircraft after it landed safely in Boston, and the captain was taken to a hospital for medical treatment.

Air Canada said it is working to arrange alternate travel for the passengers.

McDonald said he is simply thankful that the frightening flight did not end in tragedy.

“We’re just grateful,” he said. “Grateful for how it all ended.”