Emmanuel Macron slapped across face during southeast France stop of nationwide tour
Images on social media and broadcast on the BFM news channel showed Macron approach a barrier to greet a man who, instead of shaking hands, slapped the 43-year-old across the face.
Macron's bodyguards quickly intervened and two people were arrested afterwards, local officials said.
"The man who tried to slap the president and another individual are currently being questioned by the gendarmerie," the regional prefecture said in a statement.
The incident in the village of Tain-l'Hermitage in the Drome region represents a serious security breach and overshadows the start of Macron's tour which he said was designed to "take the country's pulse."
"Around 1:15 pm [11:15 am GMT], the president got back into his car after visiting a high school and came back out because onlookers were calling out to him," the prefecture said.
"He went to meet them and that's where the incident happened," it added.
The centrist is widely expected to seek re-election in next year's presidential elections and polls show him with a narrow lead over far-right leader Marine Le Pen.
Around a dozen stops had been planned over the next two months, with Macron keen to meet voters in person after more than a year of crisis management linked to the Covid-19 pandemic.
Shortly before being slapped, Macron had been asked to comment on recent remarks from leftist leader Jean-Luc Melenchon who suggested at the weekend that next year's election would be manipulated.
"Democratic life needs calm and respect, from everyone, politicians as well as citizens," Macron said.
In July last year, Macron and his wife Brigitte were verbally abused by a group of protesters while taking an impromptu walk through the Tuileries gardens in central Paris.
"Politics can never be violence, verbal aggression, much less physical aggression," Prime Minister Jean Castex told parliament after the latest incident.
Opinion - "Macron is whipping up anti-Muslim sentiment in order to cover up for his own failures: An economic crisis, rising inequality and a public health emergency", @mobbiemobes writes https://t.co/3PpPKjRuTj— The New Arab (@The_NewArab) April 4, 2021
Macron has undertaken several other tours since his 2017 electoral triumph over the traditional parties of government on the left and right.
A 2018 trip to mark the centenary of the end of World War I is best remembered for the scenes of furious citizens booing and heckling France's youngest post-war leader.
It took place just as "yellow vest" protests were gathering momentum to denounce the government's policies and the head of state personally for his leadership style, which was criticised as aloof and arrogant.
Macron conducted another tour billed as a listening exercise in 2019 in the aftermath of those protests, which shook the country and saw him promise to change his way of governing.