Two children and an adult are in a life-threatening condition after a knife attack in a park in Annecy in southeast France, police have said.
Four children and an adult were injured in total, officers added.
French President Emmanuel Macron said those badly hurt were “between life and death”.
The youngsters are all about three years of age, police said.
A man who spoke to broadcaster BFMTV said he saw first aiders working on “little bodies, three or four years old, perhaps”.
Another witness who spoke to the same channel said he saw the attacker assault an elderly man, jumping on him and stabbing him repeatedly.
He said he yelled at police to act.
“It’s a place where babysitters and parents take young children to play, and the atmosphere is fantastic,” said Yohan, who works at an ice cream parlour just opposite the park.
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3:51
Sky’s John Sparks has the latest
Interior Minister Gerald Darmanin said a suspect had been arrested “thanks to the swift intervention of police”.
The perpetrator is a Syrian man with “legal refugee status”, police said.
He is said to be in his 30s. He was not known to the French security services and his motivation is unclear, a source told the Reuters news agency.
Video footage shows two members of the public trying to apprehend him.
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Minute silence held for Annecy victims
BFMTV showed footage of several policemen overpowering an individual in the park.
“He jumped (in the playground), started shouting and then went towards the strollers, repeatedly hitting the little ones with a knife,” a witness who gave his name as Ferdinand told BFMTV.
“Mothers were crying, everybody was running,” said another witness, George, who owns a nearby restaurant.