Police and firefighters have been seen in London Road, with a stretch of pavement taped off by police officers across from the junction with Mayfield Road.
A red tent has also been set up inside the police cordon.
Firefighter Garth Waterman from Cosham Fire Station said: ‘All we know is that they’re going to be there for substantial amount of time.
‘This is a sensitive incident so we can’t give any more information at this time.’
Hampshire Constabulary has also been contacted by The News for comment.
More to follow.