The big screen reenactment of a US mountain climber cutting off his own arm with a blunt pen knife has shocked the Toronto International Film Festival, reportedly causing several people to faint in the audience.

English filmmaker Danny Boyle's 127 Hours documents the real-life dilemma of 27-year-old adventurer Aron Ralston, who became pinned by a boulder while on a canyoning trip in Utah in 2003. Facing starvation and certain death, he chose to break and then amputate his right forearm in order to escape and survive

The film was previewed at Toronto this week, with the amputation scenes hailed as "the most realistic ever put on film."

So much so, three cinema-goers fainted and another suffered a seizure, according to film news website The Wrap. Festival organisers have refused to comment on the incident.