The 18-year-old suffered a small injury to his back needing stitches at the incident near Portsmouth Naval Memorial near the seafront.
‘The initial report was of a group fighting and a bike being thrown around,’ a Hampshire police spokesman said.
Police said the incident happened between 8pm-8.30pm last night and involved ‘around 30 people’
The spokesman said: ‘Officers are trying to establish what happened and are especially keen to hear from any passing motorists who may have recorded dashcam footage of the incident.
‘Anyone who saw what happened, or who has video footage that could help our investigation, is asked to phone 101 quoting 44210233631.
‘Alternatively, you can call Crimestoppers 100 per cent anonymously on 0800 555 111.’