Six passengers were injured from the turbulence, but none of the injuries are life threatening.
"There were three big jumps. I was sitting one row behind the bathroom and I saw a woman fall and she twisted her ankle," said passenger Hayley Prickett.
The sudden turbulence so severe, some on board said it felt like an earthquake.
"Out of nowhere, it rattled up and down, and a few people came down on their legs pretty bad," said passenger Mike Hartogs.
The Boeing 777, filled with nearly 200 passengers, had left London's Heathrow Airport Tuesday morning en route to Los Angeles. It was flying over the Atlantic Ocean when turbulence suddenly struck the aircraft. The seat belt sign was off, and several passengers were severely injured.
"I felt like the plane was hit by a bus. One of the stewardesses spilled coffee all over and burned herself badly," said passenger Richard Wall.
An L.A. plastic surgeon who was on the flight jumped into action to treat the injured, including one flight attendant, who severely fractured her leg.
"She was shivering, and she was losing a lot of blood," said Dr. Semira Bayati.
The flight made an emergency landing at Montreal's Trudeau International Airport just after noon local time. Ambulances rushed to the tarmac to treat the injured, and half a dozen were taken to area hospitals.
"When I got up for a couple of minutes to go to the ladies room, that's when I saw that there were people really injured in the back, laying on the floor, lots of food spilled all over the floor," said passenger Romaine McCoi.
Exhausted and more than eight hours late, passengers finally arrived at LAX aboard a new plane, a little shaken up, but otherwise OK.
The flight crew had originally planned to make their emergency landing in Pittsburgh or Chicago, but due to the injuries, they decided to touchdown at the closest airport, which was Montreal.
The Associated Press contributed to this story.