In a video circulating on social media, Macron reached out his hand to greet a man in a small crowd of onlookers standing behind a metal barrier as he visited a professional training college for the hospitality industry.
The man, who was dressed in a khaki T-shirt, then shouted "Down with Macronia" ("A Bas La Macronie") and slapped Macron on the left side of his face.
Two of Macron's security detail tackled the man during the incident, and another ushered Macron away. Another video posted on Twitter showed that the president, a few seconds later, returned to the line of onlookers and resumed shaking hands.
Two people were arrested, a police source told Reuters. The identify of the man who slapped Macron, and his motives, were unclear.