Police were called to Copnor this afternoon in response to a call from 46-year-old transport manager.
Armed officers were sent to the incident – which is not terror related – at around 3.45pm.
Mum-of-two Rebecca Barrett, from Copnor, saw the armed officers jump from a car and pin the suspect to the bonnet of the vehicle.
The 33-year-old was walking home with her two children, aged five and three, on Burrfields Bridge when the incident happened.
She said: ‘I was showing my son the unmarked police car when two officers with huge guns got out and grabbed a man standing near us.
‘The held him against the bonnet of their car and asked me to cross the road.
‘But it was too busy and I had to stand there.’
After 10 minutes, Rebecca was allowed past and said she saw the weapon on the floor.
She added: ‘I don’t know a lot about guns but there was one on the floor near where the man was standing.’
Another witness, the transport manager, who asked not to be named, was just 6ft away from the suspected gunman who walked past him in Dundas Lane, with a revolver-type handgun.
The witness said his only thoughts were to urgently call police – with them minutes later arresting the suspect.
A 26-year-old man, from Southsea, is still in custody. Police say there is nothing to suggest the incident is terror related.
It’s not thought the weapon was capable of firing live ammunition.
The worker said: ‘A person walked past me and pulled out a revolver-type hand gun right in front of me and then put it into his pocket.
‘I then let him get a few paces away before calling 999.
‘Shortly after the men and women of Hampshire armed response team were in the area and I was then informed by them they had caught the individual.’
He added the man was wearing a yellow hoodie.
‘The gun was a revolver possibly a Colt but without actually seeing it close up I wouldn’t be sure it may have been a replica,’ he said.
‘I was in the army so I do know a little about what a gun looks like.
‘I spoke to a female officer part of the tactical armed response team.
‘He was about 6ft away from me as he walked past where I was standing.
‘My only thought was to get the police out as soon as possible.’
The first witness who called 999 praised police for their actions.
He said: ‘They are the real heroes in any situation like this putting themselves in the face of danger to enable us all to go about our business safely.’
A Hampshire police spokesman said: ‘Police are investigating reports of a man allegedly carrying a firearm in the Dundas Lane area of Portsmouth.
‘The incident took place between 3.45pm and 4.45pm today (Wednesday 7 June).
‘A 26-year-old man from Southsea has been arrested in connection with this incident and remains in custody at this time.
‘We would like to reassure members of the public that there is no evidence to suggest this is terror related.’
Anyone with information should call police on 101 quoting 44170215607.