A man's body has been discovered in waters opposite the St Kilda Marina as thousands party at the St Kilda Festival on Sunday afternoon.

Police are on the scene after a passer-by located the partially-submerged body about 1.20pm.

The man's body was pulled from the water by onlookers before the police arrived.

His cause of death is yet to be determined and police will prepare a report for the Coroner.

This comes as 400,000 revellers partied at the 38th St Kilda Festival despite the extension of an alcohol ban on the foreshore.

The ban will be in force until April 3 (the day after Easter Monday).