The attack in a busy and well-heeled neighbourhood of Paris shocked residents, though police have noted violent disputes between rival bands of youths in the area.
The detained suspects – eight minors and one young adult – were being investigated for attempted murder, gang violence and theft, prosecutors said.
Nataliya Kruchenyk, mother of the 15-year-old victim named only as Yuriy, said this week he had emerged from a coma induced by doctors following the January 15th attack near a shopping centre in the 15th arrondissement of the capital.
Kruchenyk said her son had left school with friends when he was accosted by around a dozen people, a scene captured by a surveillance camera.
The attackers, some hiding their faces with hoods, kicked and punched him as he lay on the ground, and also struck him with what appeared to be a tool.
Yuriy was carrying a screwdriver in his pocket when he was attacked, prosecutors said on Thursday.
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Lawmakers and celebrities including footballer Antoine Griezmann and actor Omar Sy of the hit series “Lupin” denounced the assault, which police said might have resulted from a gang rivalry.
Justice Minister Eric Dupond-Moretti has vowed that “there will be no impunity for Yuriy's aggressors”.
President Emmanuel Macron has promised to reduce crime and insecurity while bolstering investments for police.