A police officer was allegedly "ragdolled" and had his head smashed into the ground while another was seriously injured when a "pack of animals" attacked the pair during an arrest, police say.

The horrific assault occurred after police were called to Fitzroy St, St Kilda, just before 3am on Sunday after reports of an attack on a "vulnerable" woman.

Superintendent Tony Silva said when police arrived, a 26-year-old man who was believed to be a person of interest fled down the street.

Police said the officers ran after him and had him on the ground to arrest him when one female and five to six males in their early 20s tried to intervene.