Festival goers in Melbourne's west were punched and kicked, and had phones, wallets and bags stolen, as a gang of up to 40 youths terrorised an event in Melbourne's west on Saturday night.

Crowds at the Summersault Festival in Caroline Springs estimated about 30 to 40 teenagers of African descent went on a rampage through the event grounds about 10pm, as families gathered to watch the fireworks.

Jack McLaughlin, 16, was hospitalised after being jumped on and punched by a group, before his phone was stolen.

His mother, Debbie Tesoriero, told the Herald-Sun he has no recollection of the attack.

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"He was just an innocent bystander," Ms Tesoriero said.

"We're very fortunate it wasn't worse. As a parent this is terrifying."

Festival-goer Joe Cuic described the rampage as "hectic".

"Out of nowhere, everyone's gone from standing there watching, having a lovely time, to people just running in all directions," he said.

He told Seven News he saw some of the youths throwing rocks at a nearby apartment building after leaving the festival grounds.

"They realised we were not police and just turned around and started mouthing off to us. Give us the finger and, yeah, just couldn't care less," Mr Cuic said.

Victoria Police said six people had reported having their phones stolen and two had reported being assaulted, however, the numbers are believed to be much higher.