An Air France flight AF83 from San Francisco to Paris has been diverted to Montreal's Trudeau Airport after a bomb threat.

Pierre-Elliott-Trudeau Airport’s spokesman François-Nicolas Asselin told Le Journal de Montreal newspaper there are 231 passengers and 15 crewmembers on board. The airport is in contact with Air France and the “investigation is on” to find out more.

Following anonymous threat, crew of #AF083, decided per safety regulations to land in Montreal as a precaution. Flight landed normally 1/2 — Air France Newsroom (@AFnewsroom) December 8, 2015

ALERT Air France #AF83 from SFO to Paris CDG diverted to Montreal. Police on scene investigating /@TomPodolec — AirLive.net (@airlivenet) December 8, 2015

One of the passengers aboard has tweeted a picture from inside the plane showing fellow travellers getting ready to exit the aircraft.

UPDATE #AF83 Passengers will disembark by group of 50 to be checked individually as well as the plane /@tservalpic.twitter.com/feDb53mWQk — AirLive.net (@airlivenet) December 8, 2015

The flight departed on Monday, about 3pm San Francisco time, and was due to arrive in Paris on Tuesday at 10.30am.

At least 15 fire trucks and police cars were waiting for the aircraft and Canadian authorities were checking the plane, passengers and baggage.

Air France said in a statement Tuesday morning that the crew of Flight 83 decided to divert "as a precautionary measure,” as quoted by AP. Airline spokeswoman Ulli Gendrot said all passengers were off the plane within several hours of landing.

Passengers were allowed to leave the plane in groups of 50, and are being questioned by investigators.

Update DIVERSION Air France #AF83 San Francisco to Paris diverted to Montreal. (Fred Simon photo). Police on scene pic.twitter.com/ONea1YAier — Tom Podolec CTV News (@TomPodolec) December 8, 2015

Passenger Gilles Raymond of San Francisco told AP that after about five hours in the air the pilot announced they would have to land due to technical reasons. Once the aircraft had landed, the pilot said authorities would have to do a security check of the plane and passengers and that everyone would have to disembark.

"Everyone is pretty relaxed. There was no panic, no stress. Everyone is waiting in their seats. Some people are sleeping, and some are checking their phones or eating ice cream that the flight attendants gave them," Raymond said while still inside the aircraft.

VIDEO: Air France flight #AF83 SFO to Paris diverted to Trudeau Airport. Passengers still on place. @CTVMontrealpic.twitter.com/edj4G4MPxu — Cosmo Santamaria (@cosmoCTV) December 8, 2015

He said passengers spent at least an hour while the checks went on.

"They told us they will check every passenger and that we may spend the night here," Raymond added.