The Christian Film & Television Commission describes Django Unchained as "ultra-violent".
According to the group, the movie shows excessive amounts of blood during shooting scenes. It also criticises the depiction of a slave being eaten by dogs and a man hanging upside down being threatened with castration.
The commission is challenging the R rating at cinemas in the US, which allows children to view the film if they are accompanied by a parent or other adult, to NC-17, which prohibits anyone below the age of 17 from seeing the film.
The group is particularly alarmed by the relaxed rating in light of the recent shooting rampage in Connecticut.
It is petitioning the Motion Picture Association of America to change the rating to minimise the risk of children emulating the violence they see in films.
"This movie ends with two of the most violent scenes we've ever seen in more than 27 years of reviewing movies," said the group's founder Dr Ted Baehr.
"As countless research studies and recent events in Connecticut have shown, some young boys and men like to imitate the violence they see in movies, TV, and video games.
"Rating Django NC-17 would be the right thing to do."
Django star Jamie Foxx recently spoke about the connection between violent movies and violence in society.
"We cannot turn our back and say that violence in films or anything that we do doesn't have a sort of influence," Foxx said. "It does."
Dr Baehr continued: "There are many confused parents out there. Public safety demands that ignorant or irresponsible filmmakers not be allowed to corrupt children with graphic violence or explicit sexual images and content."