Schoolchildren set off fireworks on Fargate in Sheffield city centre
The youths - Asian boys thought to be around 13 years old - set off the fireworks near Boots.
Shoppers and city centre workers spotted the youths at around 8am.
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Hide AdEllen Barr, aged 29, confronted the boys and warned them that they were putting lives at risk.
The incident has been reported to South Yorkshire Police and CCTV footage is to be examined in a bid to identify the culprits.
Mrs Barr said: "I couldn't believe what I was seeing - it was 8am in a busy city centre - they could have hit anyone.
"I shouted over at them they could kill somebody and they just swore at me and walked off.
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Hide Ad"I heard the screech of the fireworks and saw them letting them off into the air. I wouldn't say they were targeting anyone in particular but they could easily have hit somebody."
Anyone with information should call South Yorkshire Police on 101.