Anthony Le Tallec was jogging around the pristine waters of Lake Annecy in the foothills of the Alps when a wave of panicked bystanders rushed past in the opposite direction.
One mother urged him to turn and run.
"There's someone stabbing everyone along the lakeshore. He's knifed children," Le Tallec, a former professional football player with Liverpool FC, quoted her as telling him in a video account of what he saw that he posted on Instagram.
Confused, Le Tallec said he kept jogging through the park in the town of Annecy but soon saw a man heading his way with police officers in pursuit but struggling to catch up.
"I see that he's heading straight for a group of elderly men and women. He attacks one grandpa, stabs him once, the cops can't catch him, so I tell the cops, 'Shoot him'," Le Tallec continued.
"Then they start shooting, they shoot at the person, right in front of me, and he falls to the ground."
The suspect was a 31-year-old Syrian refugee granted asylum in Sweden 10 years ago and in France legally, Prime Minister Elisabeth Borne said.
A woman identified as the suspect's former wife told BFM TV her former partner was a Christian.
One eyewitness' video of the attack, verified by Reuters, shows the assailant jump a low wall into a children's playground and repeatedly lunge at a child's stroller, pushing aside a woman who tries to fend him off.
Belgian national Laurent Syryn told Flemmish broadcaster VRT's Radio 2 that he saw two small children in a single pushchair with knife wounds.
"One had a knife wound in the neck, another in the abdomen," he said. "I applied pressure to the wounds and waited for the emergency services."
The children were wounded in the attack were aged between 22 months and three years.
Le Paquier park where the attack took place is popular with local people and tourists who swarm to Annecy in summer to boat on the lake's turquoise waters and hike in the nearby forested mountains.
At least one of the wounded children was in a stroller, eyewitnesses said. One person who saw the attack, who gave his name as Ferdinand, told BFM TV: "(He) went towards the strollers, repeatedly hitting the little ones with a knife."
Dominique Griziaux said he was at the nearby "Pop Beach" when the wail of sirens filled the air. The attack had left him feeling sick inside.
"You put yourself in the place of parents and grandparents put through this drama," said Griziaux, an Annecy resident, adding that an attack on such young children was "just unthinkable".
"You'd never think this could be possible in such a peaceful, bucolic environment as Annecy."
As the afternoon faded and life returned to the re-opened park, Pierre, 39, pushed his son on a swing in the playground where the attack occurred.
"I thought 'maybe it's better to go to another park'. But he likes it here," Pierre said.