Hundreds of youths brawl at Melbourne's St Kilda beach

Gangs of youths have wreaked havoc in Melbourne, allegedly beating and robbing beachgoers at St Kilda Beach.

A large gang is wanted over a series of assaults and robberies around the beach on Wednesday night, according to police.

Witnesses say police were left outnumbered by the hundreds of youths involved in the attacks.

Alex Carter says his 16-year-old son and his friends were punched in the face and robbed in the brazen attacks, with youths stealing their phones, watches and shirts.

"They left them standing half nude in the street," the boy's father Alex Carter told 7 News.

"These guys are so clever the way they do it (the robberies), that it just sort of looks like a couple of teenagers having a bit of fun."

Cellphone video from the beach shows youths fighting in front of beachgoers and one girl punching another in the head.

A McDonald's was also attacked in the rampage.

Police wouldn't say how many were attacked and robbed and called on the perpetrators to come forward.

"Their behaviour was just totally unacceptable and I'd call on them to come forward and hand themselves in before we track them down," Inspector Jason Kelly told reporters on Thursday.