The tragic incident was witnessed by a 13-year-old schoolboy.
The boy told The News that he was on the way home from visiting friends when he saw the man standing at the top of the multi-storey car park near Tesco in Crasswell Street.
The boy, a pupil at nearby St Edmunds School, said: 'I was walking along then I saw him shouting down from the corner on top of the car park.
'The man was shouting "Help me please, Help me God", then he shouted "Please help me save my baby", and jumped off.
'I couldn't believe it, there was a big thud.
'He did not have to jump – it's such a waste of life.'
Members of the public called the emergency services and paramedics arrived just before 12.30pm.
However, the man was pronounced dead at the scene.
Police said they were not treating his death as suspicious.
Craig Smith, 18, of Eastern Road, Portsmouth, was cycling behind the car park shortly after the man fell.
He said: 'I came around the corner and three or four people were surrounding a middle-aged man on the floor.
'He was face down and I could see his face.
'It was not very nice at all and it was a disturbing thing to see in such a busy place.'
The area of Crasswell Street by the eastern corner
of the car park was sealed off as forensic detectives examined the scene.
Later Colas contractors covered the
thoroughfare with sand.
A spokeswoman for Hampshire police
said: 'A man was involved in a fall from the car park in Crasswell Street.
'Police and ambulance attended but
the man was pronounced dead at the scene.'
The identity of the man has not been released as he has yet to be formally identified.