The caller, who fled from the scene 'fearing for their safety', reported seeing at least one of the attackers with a machete-style knife.
But when armed police arrived at the scene they found four boys - aged 12, 14, 15 and 17 - who admitted to have acted out a number of violent scenes in the city centre as a Halloween prank.
Chief Inspector Lydia Lynskey said: “All four boys have been spoken to and advised about their behaviour last night, which caused a considerable amount of concern for other members of the public.
“While they might have thought their actions were a bit of fun, they acted in a violent manner and used mock weapons on a busy street in the city centre, which would understandably cause distress and fear to passers-by and witnesses.
“Upon receiving a report of this nature, where a life is possibly at risk and weapons are seen, we would always look to deploy our specialist armed resources.
“In this case, our armed officers were diverted from their responsibilities to deal with a Halloween prank.
“This is not best use of police time and I’d warn anyone considering engaging in similar sorts of pranks this evening and across the Dark Nights period, you will be dealt with accordingly and could be in for a bit of a fright if our armed officers show up.”