Four children, including a British national, and an adult were injured in the attack in the town of Annecy in the French Alps.
At least three of the victims are in a critical condition in hospital, according to reports.
A man armed with a knife entered a children's playground at 9.45am and attacked a group of children around three years old.
Ex-Liverpool man Le Tallec, who also played for FC Annecy, was running in the park at the time and described what he saw.
"He wanted to attack everyone. I moved away and he lunged at an old man and woman and stabbed the old man," he told local Dauphine Libéré newspaper.
Le Tallec later said in on Instagram that he was surprised but kept running, later seeing the attacker chased by police.
He also said he saw him rush towards a group of elderly people and attack in old man.
The suspect was later arrested by Police and is said to be a Syrian man in his early 30s who was granted refugee status in Sweden in April.