The 20-year-old victim was set upon by four to five men in Arundel Street, Landport, at around 5.50am on Monday.
It happened next to St John’s Cathedral Catholic Primary School. The man suffered bruising to his face and minor injuries to his head and an ear.
He was taken to Queen Alexandra Hospital for treatment and has since been discharged.
One of the attackers is described as white, 5ft 8in tall, and skinny with messy curly brown hair. He was wearing a tracksuit and spoke with a Pompey accent.
A police spokeswoman said: ‘We would like to hear from anyone who was in the area at the time and may have seen or heard the altercation taking place or saw the group or the victim before or in the aftermath of the assault.
‘We believe a couple of members of the public assisted the victim after the assault.
‘If this was you please get in touch. Perhaps you were driving through the area at the time and have dash cam footage that may have captured something?’
Call police on 101 quoting 44210437738.