Hundreds of Juventus soccer fans are being treated for injuries after mayhem broke out on Saturday night in the Italian city of Turin during a public viewing of the Champions League final.
The noise triggered panic among thousands of fans gathered in Piazza San Carlo to watch the match pitting Juventus against Real Madrid on giant TV screens.
At least one person was seen being carried away on a stretcher.
He said some reported having heard a small explosion, others a shot.
"The root cause of this was panic, to understand what triggered it we will have to wait a while", said top local official Renato Saccone, the prefect of Turin.
Tensions flared during the second half of the match, which Juventus went on to lose 1-4.
For some Juventus fans, the incident brought back traumatic memories of the 1985 Heysel stadium disaster, where 39 people, majority Italians fans, were killed when a wall collapsed on them before the European Cup final against Liverpool. Two were reportedly in serious condition.
The square was evacuated so quickly it was left strewn with sneakers ripped off people's feet as they ran. About seven people were seriously hurt, including a seven-year-old boy who was trampled.
Italian media has speculated it may have been a firecracker.
Police set up an information point to help people find their loved ones, including many children who had been separated from their parents.