Lives were put at risk when a gang of youths threw a gas canister on to a bonfire, police say.
It happened at about 1900 GMT on Tuesday on the Meadow Farm recreation area in Shrewsbury.
Officers had been called to investigate reports that a group of youths were ripping up fences to burn on the bonfire.
They found about 40 youngsters, some of them thought to be as young as 12.
Petrol was also seen being thrown on to the fire.
Police said the gas canister could have killed someone if it had exploded.
In a separate incident, in nearby Harlescott Grange, fire crews had to ask for police support as they were abused by a gang of about 20 youths.