Ten people aboard a United Airlines flight heading from London to Los Angeles were treated for injuries. (CBC)

Ten people were injured after a United Airlines plane hit turbulence over the Atlantic Ocean and was forced to make an emergency landing in Montreal on Tuesday.

The flight took off at 5:05 a.m. from Terminal 1 at Heathrow Airport in London, England, and was caught in severe turbulence, the airline said.

The plane with 196 passengers was diverted to Montreal's Trudeau International Airport because of injuries on board.

The Los Angeles-bound Flight 935 touched down at 12:10 p.m. local time.

Six people were taken to hospital for treatment.

One flight attendant was treated for an ankle injury, while one passenger was treated for a broken arm, said ambulance officials. A second passenger, a pregnant woman, was suffering from abdominal pains. The extent of other injuries are not immediately known.

The passengers remained calm despite the commotion, said Benoît Garneau, a spokesman for the local ambulance service, Urgences Santé.

"They told me that everybody was calm inside, [nobody] was in shock," he said.

Investigators with the Transportation Safety Board will check the aircraft's data recorder for more information.

United Airlines' officials were also expected to inspect the Boeing 777 to ensure it was not damaged.

Turbulence severe enough to cause injury to passengers is a rare occurrence, said Joanne Richards, a flight attendant trainer with Air Transat.

"For these injuries, it has got to be unpredicted," she said. "That could do with air pressure, wind, the jet stream, [or] the temperature.

"It's a very hot day today as everyone has noticed."

Uninjured passengers were transferred to another flight brought in from Chicago on Tuesday afternoon.