Well-wishers laid flowers on Friday at the site in the French mountain town of Annecy where an attacker stabbed four toddlers, as a shocked nation paid tribute to a backpack-wearing student who tried to stop the assault.

Two of the children were in a critical but stable condition in hospital a day after the attack, Prime Minister Elisabeth Borne said. The other two - and two pensioners who were also wounded - had less serious injuries, officials said.

President Emmanuel Macron, who called the attack an "act of absolute cowardice", headed to the nearby city of Grenoble on Friday where three of the four are being cared for.

Among the children wounded in the attack where a British national and a Dutch national.

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 tried to fend him off.

France hailed the bravery of a young Catholic pilgrim who came face-to-face with the assailant and used his backpack as a shield as he sought to block the attack. French media dubbed the 24-year-old "the backpack hero".

The management and philosophy student has identified himself only as Henri.

"All I know is I was not there by chance," he told the CNews television network. "It was unthinkable to do nothing ... I followed my instincts and did what I could to protect the weak."

His Facebook and Instagram accounts were flooded with messages giving thanks for his bravery.

"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.

A mass will be held in Annecy Cathedral in tribute to the victims and their families later on Friday, church authorities said.

Police have arrested a 31-year old Syrian national who was granted asylum in Sweden 10 years ago, Borne said on Friday.

He had entered France legally, she said, and was carrying Swedish identity documents and a Swedish driving licence. Sweden and France are both members of the Schengen Area, the world's largest passport-free zone which allows the unrestricted movement of people between 26 European countries.

The attack has laid bare the tensions between Europe's free movement rights and the pressure governments in countries like France and Italy are coming under from voters to toughen immigration laws as societies shift rightwards politically.

One of the two pensioners caught up in the attack told Reuters he had been sitting on a park bench when the attacker approached on the run.

"He probably wanted to create one more victim," said Youssouf, who asked to withhold his family name.