Hampshire Constabulary say they were called at around 5.30pm on Wednesday to reports of an assault by the train station in Regent Street.
A woman in her 70s had been grabbed by a man she did not know. The victim screamed and the man ran off down a set of stairs onto Landguard Road, which exit near the holiday park and Lidl supermarket.
The man is described as around 6ft tall, broad, wearing a dark top with the hood up, a dark mask, dark trousers and light-coloured trainers.
Officers investigating the incident are now asking for a significant witness, who may have information about an incident, to come forward.
The victim walked past a dog-walker – described as a woman wearing a bobble hat, possibly blue in colour, walking a black and white dog. Police want to identify and speak to this dog-walker to see if she saw anything, or saw a man matching the description provided running down Landguard Road.
Inspector Rob Abel said:
“The victim in this case was uninjured during this incident, but was left understandably shaken by what happened.
“Officers will be carrying out enquiries in the area, but we will also have officers conducting patrols over the next few days. If you see us and have any concerns, or information, do not hesitate to approach us.
“We want to urgently identify the dog-walker to see if she perhaps saw this man or anything significant that could help us.
“Was this you? Do you know someone matching this description who walks their dog in this area?
“We understand there were vehicles passing through the area at this time. We would like to hear from anyone who drove through this area, including the lower part of Regent Street approaching the train station, between 4.45pm and 5.30pm and perhaps saw the man involved in this incident or captured anything on Dash Cam.”
Anyone with information is asked to contact police on 101, quoting 44210014346. Alternatively, you can contact the independent charity Crimestoppers 100% anonymously on 0800 555 111, or via their anonymous online form at crimestoppers-uk.org