Pictures from the scene show one boy being led away in a police van.
Two men were fighting while a third was on the ground being looked after by a woman, police said.
Hampshire Constabulary said: ‘Our officers were called out to an incident at 8.07pm last night following reports of a fracas on Southsea Common.
‘Two men were seen to be fighting while another man was laying on the ground being supported by a woman.
‘A 15-year-old boy from Southsea was arrested on suspicion of common assault, possession of an offensive weapon and possession of a class B drug.
‘He remains in custody at this time.’