It's unclear when and where the video was taken, Fox 8 reports.
In the video, the child can be heard crying before the priest slaps the baby around the 24 second mark.
Newsweek reported that the priest says, in French, that the child is having a "tantrum" and that water must be poured on his head so he could become a "little Christian". The family remained quiet for a few moments after the incident, but then the baby is grabbed out of the priest's arms by the child's father. Added another: "A priest slaps my baby and I'm putting him on the cross".
"I don't care if you're a man of the church I'm knocking you if you slap my son", wrote one person.
A baby's relatives were forced to wrestle the child away from a French priest who slapped it during baptism.
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A Malayali girl is among the children who would accompany players with the ball for the Group E match to be staged at the St. It looked as if Brazil were going to take over the match, but they couldn't muster up a consistent attack afterwards.
Onlookers, clearly appalled by what they've just seen can be heard saying, "Sir we don't hit him" and "Isn't this forbidden?"
When the watching audience begins to murmur in shock, the priest repeated: "There, you have to calm yourself, you have to calm yourself, there, there".
But the child continues to cry and the man raises his voice to say: "I will scream louder than you". After he fails to calm the child down, the priest ends up slapping the child, uttering the words "Calm down" in front of the toddler's surprised and shocked parents.
Many people online have been outraged by the priest's behaviour.