Witnesses reported seeing a convoy of police cars and ambulance descend on the rear of an address at Kingston Crescent, Landport, at 3.51pm.
A man in his 30s was found by emergency crews before being taken to Southampton General Hospital with what police have described as a ‘serious injury’.
A 20-year-old man from Portsmouth was arrested on suspicion of inflicting grievous bodily harm with intent, and possession of an offensive weapon after officers moved quickly to the area before detaining him.
He is in custody.
A police spokeswoman said: ‘A (crime) scene remains in place and there will be a police presence in the area this evening.’
An eyewitness, who did not want to be named, said: ‘I saw a young man being put in an ambulance before they drove off while being escorted by police.
‘There were about nine police cars and ambulances. The road was totally blocked off.
‘It’s scary - there are lots of problems around here. A woman was found dead in Kingston Crescent last year near this incident.’