Dog walker Karl Browning, 57, saw the police officers at around 2-2.30pm today south of the zebra crossing in Purbrook Way.
Several vehicles and officers were in the area, he said.
Karl said: ‘(I) was walking the dogs round The Oaks.
‘There were two police cars and a plain van parked up.
‘There were officers standing around, a couple had bullet proof vests and looked they were (possibly) armed.’
He added: ‘Also the river that runs through there (had) police tape following the river quite a way.’
The News has contacted Hampshire police for further information.