And then stunned witnesses say "he's afraid" and tell the priest to let go.
At first, the 43-second video clip shows the priest, who is speaking French, attempting to comfort the crying baby.
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The baby can be seen crying as the family hands him over to the priest.
A man standing behind the child, who's suspected to be the baby's father, is also seen looking shocked at the priest's actions and shortly afterward begins to grab the child away from the priest.
Warning: The contents of the video may be disturbing.
He then slaps the child on the cheek, loudly enough that the impact can be heard in the video, recorded from a couple of metres away.
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The bawling baby is being held by its mother when the elderly priest takes a grip of its head with his hand.
"I'm going to put water on the forehead, and after I'll embrace the young fellow".
"I don't care if you're a pastor, priest or one of Jesus 12 disciples", one angry viewer tweeted. "If. You". Added another: "A priest slaps my baby and I'm putting him on the cross".
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