Victoria Police has been attacked by its own union over claims junior police officers were left exposed when they responded to a violent riot early on Sunday morning that left a teenage boy with a seriously injured leg.

Police failed to deploy more officers to monitor a controversial rap night at Collingwood's Gasometer Hotel, despite urging the venue to cancel the gig due to the major threat of gang violence, the Police Association said.

But a source close to the venue disputed claims that police told the hotel to cancel the event.

Police Association secretary Wayne Gatt lashed the force for a "complete lack of planning" that put junior officers and the public at risk, saying police knew the 66 Records music event had the potential to erupt "and indeed it did".

The association also revealed that most of the officers initially sent into the fray were fresh from the academy.