France is calling him "the backpack hero", a young Catholic pilgrim on a walking tour of cathedrals who came face to face with the man who stabbed several children in a park in Annecy.

Henri, a 24-year old management and philosophy student, was near the playground in Le Paquier park when he saw the man attacking children in a stroller as their mother desperately tried to shield them on Thursday.

Video footage shows him trying to block the assailant with one of his two backpacks, pursuing the man into the playground and throwing one of his bags at the attacker.

So far, most of France only knows him by his first name. That is all there is on his social media accounts and police have not gone into more detail.

French media have simply called him "héros au sac à dos" (the backpack hero). His only public comment so far - the message "Pray for the children, I am ok," on his Facebook page.

Europe 1 Radio said Henri would meet President Emmanuel Macron later on Friday.

Four toddlers and two pensioners were stabbed in a knife attack in the tranquil French mountain town on Thursday. Authorities said the main suspect was a Syrian refugee.

A video of the attack, taken by a bystander and verified by Reuters, showed the assailant jump a low wall into a children's playground and repeatedly lunge at a child in a stroller, pushing aside a woman who tries to fend him off, while Henri pursues him and hits him with his backpack.

He is then seen pursuing the assailant across the wide expanse of the Le Paquier grasslands, dropping one of his backpacks to run faster, as police also start giving chase.

Henri's Facebook and Instagram accounts were flooded with messages giving thanks for his actions.

"May God bless you ... you did what you could at that moment, you did not give up, you did not run. You are an angel," Instagram user Mag Capone wrote on his site.